Semiconductor device and method of manufacturing the same

ABSTRACT

An objective is to provide a method of manufacturing a semiconductor device, and a semiconductor device manufactured by using the manufacturing method, in which a laser crystallization method is used that is capable of preventing the formation of grain boundaries in TFT channel formation regions, and is capable of preventing conspicuous drops in TFT mobility, reduction in the ON current, and increases in the OFF current, all due to grain boundaries. Depressions and projections with stripe shape or rectangular shape are formed. Continuous wave laser light is then irradiated to a semiconductor film formed on an insulating film along the depressions and projections with stripe shape of the insulating film, or along a longitudinal axis direction or a transverse axis direction of the rectangular shape. Note that although it is most preferable to use continuous wave laser light at this point, pulse wave laser light may also be used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device formed by usinga semiconductor film having a crystalline structure, and to a method ofmanufacturing the semiconductor device. The present invention alsorelates to a semiconductor device containing a field effect transistor,in particular, a thin film transistor, in which an island-likesemiconductor region containing a channel formation region is formed bya crystalline semiconductor film formed on an insulating surface, and toa method of manufacturing the semiconductor device.

2. Description of the Related Art

In recent years, a technique of forming a TFT on a substrate has greatlyprogressed, and its application and development for active matrixsemiconductor display devices have been advanced. In particular, since aTFT using a polycrystalline semiconductor film has higher field-effectmobility (also referred to as mobility) than a TFT using a conventionalamorphous semiconductor film, it enables high-speed operation. It istherefore possible to control the pixel by the driver circuit formed onthe same substrate where the pixel is formed, though the pixel isconventionally controlled by a driver circuit provided outside thesubstrate.

Incidentally, for substrates used in semiconductor devices, a glasssubstrate is regarded as promising in comparison with a single crystalsilicon substrate in terms of the cost. Also, a technique of forming anamorphous silicon film on an insulating substrate made of glass or thelike and using laser processing for crystallization is known. A glasssubstrate is inferior in heat resistance and is easily subjected tothermal deformation. Therefore, in the case where a polysilicon TFT isformed on the glass substrate, in order to avoid thermal deformation ofthe glass substrate, the use of laser annealing for crystallization ofthe semiconductor film is extremely effective. The thin film transistor(hereinafter referred to as TFT) formed with the crystallized siliconfilm is applied to, for example, a liquid crystal display device.

Laser annealing has characteristics such as remarkable reduction ofprocessing time compared to an annealing method utilizing radiantheating or thermal conductive heating. In addition, a semiconductor or asemiconductor film is selectively and locally heated so that a substrateis scarcely thermally damaged.

Note that the term “laser annealing method” herein indicates a techniquefor re-crystallizing a damaged layer formed in a semiconductor substrateor in a semiconductor film, a technique for crystallizing asemiconductor film formed on a substrate, and a method of improving thecrystallinity of a semiconductor film with a crystal structure(crystalline semiconductor film), for example. This also includes atechnique that is applied to planarizing or improvement of a surfacequality of the semiconductor substrate or the semiconductor film.Applicable laser oscillation apparatuses are: gas laser oscillationapparatuses represented by an excimer laser; and solid laser oscillationapparatuses represented by a YAG laser. It is known that such a deviceperforms crystallization by heating a surface layer of the semiconductorby irradiation of the laser light in an extremely short period of timeof about several tens of nanoseconds to several tens of microseconds.

One example of crystallizing an amorphous semiconductor film by usinglaser light irradiation is a technique, disclosed by Patent Document 1shown below, in which laser light is scanned at a high speed, equal toor greater than the beam spot diameter×5000/sec, thus performingcrystallization of the amorphous semiconductor film to prepare apolycrystalline semiconductor film without reaching a state wherein theamorphous semiconductor film is completely melted. A technique in whichdrawn out laser light is irradiated to a semiconductor film formed in anisland-like, substantially forming a single crystal region is disclosedin Patent Document 2 listed below. Alternatively, a method is known inwhich a beam is processed into a linear shape by an optical system likea laser processing apparatus disclosed by Patent Document 3 listedbelow, and then irradiated.

In addition, a technique of manufacturing a TFT in which the secondharmonic of laser light is irradiated to an amorphous semiconductor filmusing a solid state laser oscillation apparatus, such as an Nd:YVO₄laser, thus forming a crystalline semiconductor film having a largecrystal grain size compared to a conventionally formed crystallinesemiconductor film, is disclosed in Patent Document 4 listed below.

[Patent Document 1]

JP 62-104117 A (page 92)

[Patent Document 2]

U.S. Pat. No. 4,330,363 (FIG. 4)

[Patent Document 3]

JP 8-195357 A (pages 3-4, FIGS. 1-5)

[Patent Document 4]

JP 2001-144027 A (page 4)

Attempts of forming a single crystal semiconductor film on an insulatingsurface have been carried out from long ago, and a technique referred toas graphoepitaxy has been proposed as a very progressive attempt.Graphoepitaxy is a technique in which a step is formed in the surface ofa quartz substrate, an amorphous semiconductor film or a polycrystallinesemiconductor film is formed on the quartz substrate, heat treatment isthen performed by using a laser beam or a heater, thus forming anepitaxial growth layer with the step shape formed on the quartzsubstrate taken as a nucleus. This technique is disclosed by Non-PatentDocument 1 and the like, for example.

[Non-Patent Document 1]

“Grapho-Epitaxy of Silicon on Fused Silica Using Surface Micropatternsand Laser Crystallization”, J. Vac. Sci. Technol., 16(6), 1979, pp.1640-1643.

Further, a semiconductor film crystallization technique referred to asgraphoepitaxy is also disclosed in Non-Patent Document 2, for example.This is a technique in which epitaxial growth of a semiconductor film bythe introduction of an artificially formed surface relief grating on anamorphous substrate surface was attempted. The graphoepitaxy techniquedisclosed in Non-Patent Document 2 is one in which a step is formed inthe surface of an insulating film, and epitaxial growth of asemiconductor film crystal is attained by conducting heat treatment,laser light irradiation, or other processing on a semiconductor filmformed on the insulating film.

[Non-Patent Document 2]

M. W. Geis, et al., “CRYSTALLINE SILICON ON INSULATORS BY GRAPHOEPITAXY”Technical Digest of International Electron Devices Meeting, 1979, p.210.

Laser beams are classified into two types: pulse oscillation andcontinuous oscillation, according to an oscillation method. In the pulseoscillation laser, an output energy is relatively high, so that massproductivity can be increased by setting the size of a beam spot toseveral cm² or more. In particular, when the shape of the beam spot isprocessed using an optical system and made to be a linear shape of 10 cmor more in length, it is possible to efficiently perform irradiation ofthe laser light to the substrate and further enhance the massproductivity. Thus, for crystallization of the semiconductor film, theuse of a pulse oscillation laser is becoming mainstream.

In recent years, however, it has been found that the grain size ofcrystals formed in a semiconductor film is larger when a continuous wavelaser is used to crystallize a semiconductor film than when a pulseoscillation laser is used. With crystals of larger grain size in asemiconductor film, the mobility of TFTs formed from this semiconductorfilm is increased. As a result, continuous wave laser beams are nowsuddenly attracting attention.

A crystalline semiconductor film manufactured by using a laser annealingmethod, which is roughly divided into pulse wave and continuous wavetypes, is generally formed with an aggregation of a plurality of crystalgrains. The position and size of the crystal grains are random, and itis difficult to specify the crystal grain position and size when forminga crystalline semiconductor film. Crystal grain interfaces (grainboundaries) therefore exist within an active layer formed by patterningthe aforementioned crystalline semiconductor film into an island-like.

In contrast with the crystal grains, countless recombination centers andcapture centers exist in the grain boundaries due to an amorphousstructure, crystal defects, and the like. It is known that a carrier istrapped in the capture centers, the potential of the grain boundariesrises, and the grain boundaries become barriers with respect to thecarrier, and therefore the current transporting characteristics for thecarrier are reduced. The existence of grain boundaries within the TFTactive layer, in particular within a channel formation region, thereforeexerts a great influence on the characteristics of the TFT in which aTFT mobility drops considerably, an ON current is reduced, and an OFFcurrent is increased due to electric current flowing in the grainboundaries. Further, the characteristics of a plurality of TFTs,manufactured on the premise that the same characteristics can beobtained, may vary depending on the existence of grain boundaries withinthe active layers.

The position and size of the crystal grains obtained when irradiatinglaser light to a semiconductor film become random due to the followingreasons. A certain amount of time is required until the generation ofsolid state nuclei within a liquid semiconductor film which iscompletely melted by the irradiation of laser light. Countless crystalnuclei are generated in completely melted regions along with the passageof time, and crystal grows from each of the crystal nuclei. Thepositions at which the crystal nuclei are generated are random, andtherefore the crystal nuclei are distributed non-uniformly. Crystal growstops at points where the crystal nuclei run into each other, andtherefore the position and the size of the crystal grains become random.

It is ideal to form the channel formation region, which exerts a greatinfluence on the TFT characteristics, by a single crystal grain, thuseliminating the influence of grain boundaries. However, it is nearlyimpossible to form an amorphous silicon film, in which grain boundariesdo not exist, by using a laser annealing method. It has therefore notbeen possible to date to obtain characteristics equivalent to those of aMOS transistor, which is manufactured on a single crystal siliconsubstrate, in a TFT that uses a crystalline silicon film crystallized byemploying laser annealing.

A method of recrystallization from a melted state by heating asemiconductor film on a single crystal substrate to a high temperature,known as zone melting and the like, is in the main stream in order toform a high quality crystalline semiconductor film on an insulatingsurface having aligned orientation, few defects, few crystal grainboundaries, and few subboundaries.

A step on the base like that of a known graphoepitaxy technique isutilized, and therefore crystals grow along the step, but the step whichremains in the surface of the formed single crystal semiconductor filmis considered to be a problem. In addition, a single crystalsemiconductor film cannot be formed on a glass substrate having arelatively low distortion point by using graphoepitaxy.

In any case, it has not been possible to form a crystallinesemiconductor film in which semiconductor volumetric contraction causedby crystallization, defects due to thermal stresses with the base,lattice mismatching, or the like, crystal grain boundaries, andsubboundaries do not exist. Except a bonded SOI (silicon on insulator),it has not been possible to obtain quality equivalent to that of a MOStransistor formed on a single crystal substrate, in a crystallinesemiconductor film that is formed on an insulating surface and thencrystallized or recrystallized.

SUMMARY OF THE INVENTION

In view of the aforementioned problems, an object of the presentinvention is to provide a semiconductor device structured by a highspeed semiconductor element having high current driver performance, inwhich a crystalline semiconductor film is formed on an insulatingsurface, and the crystalline semiconductor film has as few crystal grainboundaries and crystal subboundaries as possible intersecting at leastwith a channel length direction.

Further, another object is to provide a method of manufacturing asemiconductor device, and a semiconductor device manufactured by usingthe manufacturing method, in which a laser crystallization method isused that is capable of preventing the formation of grain boundaries inTFT channel formation regions, and is capable of preventing conspicuousdrops in TFT mobility, reduction in the ON current, and increases in theOFF current, all due to grain boundaries.

The applicants of the present invention discovered that, if asemiconductor film is formed on an insulating film having depression andprojection, and laser light is irradiated to the semiconductor film,grain boundaries will then be formed selectively in portions of acrystallized semiconductor film located on projections of the insulatingfilm.

FIG. 19A shows a TEM cross sectional image in a direction orthogonal toa laser light scanning direction when continuous wave laser light isirradiated to a 200 nm thick non-single crystal semiconductor filmformed on an insulating film having depressions and projections at ascanning speed of 5 cm/sec. Reference numerals 8001 and 8002 in FIG. 19Bdenote projections formed in the insulating film. A crystallizedsemiconductor film 8004 has grain boundaries 8003 in the portions on theprojections 8001 and 8002.

The TEM cross sectional image shown in FIG. 19A is shown schematicallyin FIG. 19B. The grain boundaries 8003 are formed in the upper portionsof the projections 8001 and 8002. The applicants of the presentinvention considered that the semiconductor film disposed on an upperportion of the insulating film is volumetrically transferred toward abottom portion of a depression due to its temporary melting state causedby laser light irradiation, and therefore the semiconductor filmdisposed on the upper portion becomes thinner, to be unable to withstandstress, and grain boundaries develop therein. Although a semiconductorfilm thus crystallized has grain boundaries selectively formed in theupper portions of the projections, grain boundaries do not form easilyin portions disposed in depressions (regions denoted by dotted lines).Note that the term depressions indicates sunken regions where theprojections are not formed.

The applicants of the present invention then considered that thelocations at which the grain boundaries are formed could be determinedselectively by intentionally forming portions in the semiconductor filmat which stress will concentrate. An insulating film having depressionsand projections are formed on a substrate in the present invention, anda semiconductor film is formed on the insulating film. Portions at whichstress is concentrated are thus formed selectively in the semiconductorfilm during crystallization performed by laser light. Specifically,depressions and projections are formed in the semiconductor film.Continuous wave laser light is then irradiated along the longitudinaldirection of the depressions and projections formed in the semiconductorfilm. Note that, although it is most preferable that continuous wavelaser light be used at this point, pulse wave laser light may also beused. Note also that the cross section of the projections in a directionorthogonal to the scanning direction of the laser light may berectangular, triangular, or trapezoidal.

Grain boundaries are thus formed selectively on the projections of thesemiconductor film during crystallization by laser light with thisstructure. Grain boundaries are relatively difficult to be formed in thesemiconductor film disposed on the depressions of the insulating film.The semiconductor film disposed on the depressions of the insulatingfilm has superior crystallinity, but does not always contain no grainboundaries. However, even if grain boundaries do exist, the crystalgrains are larger, and the crystallinity is relatively superior,compared to those in the semiconductor film located on the projectionsof the semiconductor film. The locations at which grain boundaries areformed in the semiconductor film can therefore be forecast to a certainextent at the stage when the shape of the insulating film is designed.That is, the locations at which the grain boundaries are formed in thepresent invention can be selectively determined, and therefore itbecomes possible to lay out an active layer so that as few grainboundaries as possible are contained in the active layer, preferably asfew as possible in the channel formation region.

The formation of grain boundaries in the TFT channel formation regioncan be prevented with the present invention by effectively using thesemiconductor film disposed on the depressions of the insulating filmfor TFT active layers. Conspicuous drops in TFT mobility, reductions inthe ON current, and increases in the OFF current, all due to grainboundaries, can be prevented. Note that the designer can suitablydetermine a portion to be removed by patterning as being adjacent to theedges of the projections or the depressions.

In order to solve the aforementioned problems, according to the presentinvention, there is provided a method of manufacturing a semiconductordevice, including:

forming an insulating film on a substrate that has an insulatingsurface, the insulating film having opening portions;

forming a non-single crystal semiconductor film on the insulating filmand in the opening portions;

melting the non-single crystal semiconductor film, thus performingcrystallization or recrystallization and forming crystallinesemiconductor films filling the opening portions of the insulating film;and

forming a gate insulating film and gate electrodes so that thecrystalline semiconductor films filling the opening portions aresuperimposed with the gate electrodes through the gate insulating film.

The opening portion may be formed by etching the surface of theinsulating substrate, and the opening portion may also be formed byusing a silicon oxide film, silicon nitride film, silicon oxynitridefilm, or the like, and performing etching thereon. The opening portionshould be disposed with the position of an island-like semiconductorregion containing a thin film transistor channel formation region, andit is preferable to form the opening portion to conform at least withthe channel formation region.

An amorphous semiconductor film of a polycrystalline semiconductor filmformed by plasma CVD, sputtering, or low pressure CVD, a polycrystallinesemiconductor film formed by solid state growth, or the like are appliedto the non-single crystal semiconductor film. Note that the termamorphous semiconductor film as used in the present invention includesnot only its strict definition of semiconductor films having acompletely amorphous structure, but also includes semiconductor films ina state containing micro-crystal grains, so-called microcrystallinesemiconductor films, and semiconductor films containing a locallycrystalline structure. An amorphous silicon film is typically applied.In addition, amorphous silicon germanium films, amorphous siliconcarbide films, and the like can also be applied.

Pulse wave or continuous wave laser light generated from a gas laseroscillation apparatus or a solid state laser oscillation apparatus as alight source is applied as a means of melting the non-single crystalsemiconductor film and performing crystallization. The laser lightirradiated is condensed into a linear shape by using an optical system.The intensity distribution of the laser light has a uniform region in alongitudinal direction, and may also possess a distribution in atransverse direction. A solid state laser oscillation apparatus having arectangular shape beam is applied as the laser oscillation apparatusused as the light source. In particular, it is preferable to apply aslab laser oscillation apparatus. Alternatively, a construction may beapplied in which a solid state laser oscillation apparatus using a roddoped with Nd, Yb, Tm, and Ho, in particular a solid state laseroscillation apparatus using crystals of YAG, YVO₄, YLF, YAlO₃ or thelike doped with Nd, Tm, and Ho, is combined with a slab structureamplifier. Crystals of Nd:YAG, Nd:GGG (gadolinium, gallium, garnet),Nd:GSGG (gadolinium, scandium, gallium, garnet), and the like can beused as the slab material. A zigzag light path is followed with a slablaser, while total reflection is repeated within the platy shape lasermedium.

Further, strong light corresponding to the laser light may also beapplied. For example, light having a high energy density condensed byusing a reflective mirror, a lens, or the like from light emitted by ahalogen lamp, a xenon lamp, a high pressure mercury lamp, a metal halidelamp, or an excimer lamp may also be used.

The expanded laser light condensed into a linear shape or the stronglight is irradiated to the non-single crystal semiconductor film, andthe laser light irradiation position and the substrate on which thenon-single crystal semiconductor film is formed are moved relative toeach other. The laser light is scanned over a portion of the substrateor the entire substrate, thus melting the non-single crystalsemiconductor film and performing crystallization or recrystallization.It is preferable to perform laser scanning in a direction along thelongitudinal direction of the opening portions or the longitudinaldirection of the channel formation regions. Crystal thus grows along thelaser light scanning direction, and crystal grain boundaries and crystalsubboundaries can be prevented from intersecting the channellongitudinal direction. Note that, the present invention is not alwayslimited to this.

A semiconductor device of the present invention manufactured asdescribed above is characterized by including:

a substrate having an insulating surface;

an insulating film formed on the substrate and having an openingportion; and

a crystalline semiconductor film formed on the substrate and having aregion that fills the opening portion and a channel formation region inthe filled region.

According to another structure of the present invention, a semiconductordevice is characterized by including:

a substrate having an insulating surface;

an insulating film formed on the substrate and having an opening portionextending in a channel longitudinal direction; and

a crystalline semiconductor film formed on the substrate and having aregion that fills the opening portion and a channel formation region inthe filled region,

in which the opening portion has a depth equal to or greater than thedepth of the crystalline semiconductor film.

According to another structure of the present invention, a semiconductordevice is characterized by including:

an insulating surface having an opening portion extending in arectangular shape or a stripe shape;

a crystalline semiconductor film formed in the opening portion;

a gate insulating film; and

a gate electrode overlapped with the crystalline semiconductor filmthrough the gate insulating film.

According to another structure of the present invention, a semiconductordevice is characterized by including:

an insulating surface having an opening portion extending in a channellongitudinal direction;

a crystalline semiconductor film formed in the opening portion;

a gate insulating film; and

a gate electrode overlapped with the crystalline semiconductor filmthrough the gate insulating film.

By making the depth of the opening portion on the same order as, orgreater than, the semiconductor film thickness, the semiconductor meltedby the laser light or the strong light will aggregate and solidify inthe opening portion (that is, depression portion) due to surfacetension. As a result, the thickness of the semiconductor film in theopening portion (that is, projection) becomes thinner, and stressdistortion can be made to concentrate there. Further, side surfaces ofthe opening portion have an effect to a certain extent for limiting thecrystal orientation. The angles of the side surfaces of the openingportion are formed at 5 to 120° with respect to the substrate surface,preferably at 80 to 100°.

After the semiconductor film is melted by irradiation of the laser lightor the strong light, solidification begins from a region at which thebottom surfaces and the side surfaces of the opening portion intersect,and crystal growth begins from here. For example, results of performingthermal analysis simulations at points A, B, C, and D in a system areshown in FIG. 17 wherein a step shape is formed by an insulating film(1) and an insulating film (2). Characteristics are obtained as shown inFIG. 18. Heat escapes from directly underneath the insulating film (2)and from the insulating film (1) existing on the side surfaces, andtherefore the fastest temperature drop is at the point B. The points A,C, and D follow in that order. The simulation results are for a case inwhich a sidewall angle is 45°, but a qualitatively similar phenomenoncan also be considered for a case in which the angle is 90°.

That is, the semiconductor film is once placed in a melted state, itaggregates in the opening portion formed in the insulating surface dueto surface tension, and crystal growth occurs roughly from theintersection between the bottom portion and sidewalls of the openingportion. Distortions that develop accompanying crystallization can thusbe concentrated in regions outside of the opening portion. In otherwords, the crystalline semiconductor film formed so as to fill theopening portion can be freed from distortions.

Note that the energy density in the vicinity of the edge of the laserbeam of laser light is generally low compared to that near the center ofthe beam, and the crystallinity of the corresponding semiconductor filmoften inferior. It is therefore preferable that laser light scanning beperformed such that portions which later become TFT channel formationregions and the edges of the trajectory are not overlapped with eachother.

With the present invention, data (pattern information) obtained at thedesign stage on the shape of the insulating film or the semiconductorfilm as seen from above the substrate is first stored in a storingmeans. A scanning path for the laser light is then determined from thepattern information and from the width of the laser beam of laser lightin the scanning direction, and in a direction orthogonal to the scanningdirection, so that at least portions that become TFT channel formationregions are not overlapped with the edge of the laser light trajectory.The substrate position is then disposed using a marker as a reference,and laser light is irradiated to the semiconductor film on the substratein accordance with the determined scanning path.

With the aforementioned structure, the laser light can be irradiatedonly to indispensable portions, not to the entire substrate. Timerequired to irradiate the laser light to unnecessary portions cantherefore be omitted, the time needed for laser light irradiation can bereduced, and the substrate processing speed can be increased. Further,damage caused to the substrate by irradiating the laser light tounnecessary portions can be prevented.

Note that the marker may be formed by directly etching the substrateusing laser light or the like, and that the marker may also be formed ina portion of the insulating film at the same time as the insulating filmhaving depressions and projections is formed. Furthermore, positionalignment of the substrate may also be performed by reading in the shapeof the actually formed insulating film or the actually formedsemiconductor film by using an image pickup element such as a CCD,storing the shape as data in a first storing means, storing the patterninformation for the insulating film or the semiconductor film obtainedat the design stage in a second storing means, and checking the datastored in the first storing means against the pattern information storedin the second storing means.

The number of masks used for the marker can be reduced by forming themarker in a portion of the insulating film or by using the shape of theinsulating film as a marker. Moreover, the marker can be formed in amore accurate location and the precision of the position alignment canbe increased more than by forming the marker in the substrate by usinglaser light.

Note that the energy density of the laser light is generally notcompletely uniform, but instead the energy changes by position withinthe laser beam. It is necessary with the present invention to irradiatelaser light having a fixed energy density to, at least, portions thatbecome channel formation regions, and preferably over all flat surfacesof the depressions or all flat surfaces of the projections. It istherefore necessary with the present invention to use a laser beamhaving an energy distribution such that a region having a uniform energydensity completely is overlapped, at least, with portions that becomechannel formation regions, and preferably, all flat surfaces of thedepressions or all flat surfaces of the projections. It is consideredthat the shape of the laser beam is preferably rectangular, linear, orthe like in order to satisfy the aforementioned energy densityconditions.

In addition, portions of the laser beam having a low energy density maybe blocked through use of a slit. Laser light having a relativelyuniform energy density can be irradiated to all flat surfaces of thedepressions or all flat surfaces of the projections by using the slit,and crystallization can be performed uniformly. Further, the width ofthe laser beam can be partially changed in accordance with theinsulating film or semiconductor film pattern information with the slit.The channel formation region, and in addition the layout of the TFTactive layer, can be less limitted. Note that the term laser beam widthdenotes the length of the laser beam in a direction that is orthogonalto the scanning direction.

Further, one laser beam obtained by combining laser lights emitted froma plurality of laser oscillation apparatuses may also be used in lasercrystallization. Portions at which the energy density is low in each ofthe laser lights can be supplemented with each other by using theaforementioned structure.

Further, laser light irradiation may be performed without exposure tothe outside atmosphere after forming the semiconductor film (forexample, under a specific gas atmosphere of an inert gas, nitrogen,oxygen, or the like, or under a reduced pressure atmosphere), thuscrystallizing the semiconductor film. Contaminant substances on amolecular level within a clean room, for example boron and the likecontained within a filter for increasing the cleanliness of air, can beprevented from mixing into the semiconductor film when performingcrystallization by laser light with the aforementioned structure.

Note that the conventional technique of crystallizing a semiconductorfilm referred to as graphoepitaxy is attempted for epitaxial growth of asemiconductor film induced by a surface relief grating on anartificially formed amorphous substrate. Techniques relating tographoepitaxy can be found in the aforementioned Non-Patent Document 2and the like. The graphoepitaxy is disclosed in the aforementioneddocument and the like as forming a step in the surface of an insulatingfilm, and performing heating, laser light irradiation, or the like to asemiconductor film formed on the insulating film, thus causing crystalsin the semiconductor film to grow epitaxially. However, the temperaturenecessary for epitaxial growth is, at least, on the order of 700° C. Ifepitaxial growth is performed on a glass substrate, then grainboundaries are formed in the semiconductor film in the vicinity of theedges of the depressions or the projections of the insulating film. Withthe present invention an island mask is laid out, and the shape of thedepressions or the projections of the insulating film, and its edgelocations, are designed to correspond to the island mask so as toincrease crystallinity in portions that become islands. Specifically,the shape, the size, and the like for the depressions or the projectionsare determined so that the edges of the depressions or the projections,and the vicinity of the center between the edges of the depression andthe projection, are not overlapped with the islands. The locations ofthe grain boundaries are therefore intentionally and selectivelydetermined by using the insulating film designed corresponding to theisland layout. Portions of the semiconductor film in which the grainboundaries are selectively formed may then be removed, and portionshaving relatively superior crystallinity may be used as islands. Thetechnique disclosed in the present invention therefore is in agreementwith conventional graphoepitaxy in the following points: a semiconductorfilm is formed on an insulating film in which a step has been formed;and the semiconductor film is crystallized by utilizing the step.However, the locations of the grain boundaries are not controlled byusing the step in conventional graphoepitaxy, and the concept ofreducing grain boundaries within the islands is not included inconventional graphoepitaxy. The conventional technique only resemblesand differs from the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings are as follows.

FIG. 1 is a perspective view for explaining a crystallization method ofthe present invention.

FIG. 2 is a perspective view for explaining the crystallization methodof the present invention.

FIG. 3 is a perspective view for explaining the crystallization methodof the present invention.

FIG. 4 is a perspective view for explaining the crystallization methodof the present invention.

FIGS. 5A to 5E are longitudinal cross sectional diagrams for explainingdetails of the relationship between the shape of an opening portionduring crystallization and the form of a crystalline semiconductor film.

FIGS. 6A and 6B are layout drawings showing an embodiment of a laserirradiation apparatus applied in the present invention.

FIGS. 7A to 7C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining a process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 8A to 8C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 9A to 9C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 10A to 10C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 11A to 11C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 12A to 12C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 13A to 13C are an upper surface diagram and longitudinal crosssectional diagrams, respectively, for explaining the process ofmanufacturing a TFT manufactured by the present invention.

FIGS. 14A to 14D are an upper surface diagram, longitudinal crosssectional diagrams and an enlarged view of a portion thereof,respectively, for explaining the process of manufacturing a TFTmanufactured by the present invention.

FIGS. 15A to 15C are external views of a display panel.

FIG. 16 is an upper surface diagram for explaining the structure of apixel portion of a display panel manufactured by using the presentinvention.

FIG. 17 is a cross sectional diagram showing the structure used in athermal analysis simulation.

FIG. 18 is a graph showing the results of the thermal analysissimulation.

FIGS. 19A and 19B are a TEM cross sectional image taken aftercrystallization by irradiating laser light to a semiconductor filmformed on a base film having a projection, and a schematic diagram ofthe image, respectively.

FIGS. 20A to 20C are diagrams showing the flow of crystallization of asemiconductor film of the present invention.

FIGS. 21A to 21C are diagrams showing a process of manufacturing a TFTof the present invention.

FIGS. 22A and 22B are diagrams showing the process of manufacturing aTFT of the present invention.

FIGS. 23A and 23B are diagrams showing the process of manufacturing aTFT of the present invention.

FIGS. 24A and 24B are diagrams showing the process of manufacturing aTFT of the present invention.

FIGS. 25A to 25C are diagrams showing the process of manufacturing a TFTof the present invention.

FIGS. 26A and 26B are cross sectional diagrams of a TFT.

FIGS. 27A and 27B are cross sectional diagrams of a TFT.

FIGS. 28A to 28D are diagrams showing a process of manufacturing a TFTof the present invention.

FIGS. 29A to 29D are diagrams showing a process of manufacturing a TFTof the present invention.

FIGS. 30A and 30B are upper surface diagrams of a plurality of TFTsformed on a base film.

FIGS. 31A to 31E are diagrams showing the flow of crystallization of asemiconductor film of the present invention when using a catalyticelement.

FIG. 32 is a diagram of a laser irradiation apparatus.

FIG. 33 is a diagram of a laser irradiation apparatus.

FIGS. 34A to 34D are diagrams showing a process of manufacturing a basefilm having a projection.

FIGS. 35A to 35C are diagrams showing a process of manufacturing a basefilm having a projection.

FIGS. 36A to 36D are diagrams showing the energy density distribution ofa laser beam.

FIGS. 37A and 37B are diagrams showing the energy density distributionof a laser beam.

FIG. 38 is a diagram showing the energy density distribution of a laserbeam.

FIG. 39 is a diagram of an optical system.

FIG. 40 is a diagram showing the energy density distribution in acentral axial direction of laser beams overlapped with each other.

FIG. 41 is a diagram showing the relationship between the distancebetween laser beam centers and the energy difference.

FIG. 42 is a diagram showing the output energy distribution in a centeraxial direction of a laser beam.

FIG. 43 is a diagram showing the structure of a light emitting devicewhich is an example of a semiconductor device of the present invention.

FIG. 44 is a diagram showing the structure of a pixel of the lightemitting device which is an example of a semiconductor device of thepresent invention.

FIGS. 45A to 45H are diagrams of electronic equipment that use thesemiconductor device of the present invention.

FIGS. 46A and 46B are a cross sectional diagram of a TFT having a stackstructure, and an example of the structure of a semiconductor deviceusing the TFT, respectively.

FIGS. 47A to 47C are diagrams showing the frequency distribution of theS value.

FIGS. 48A to 48C are diagrams showing the frequency distribution of thethreshold voltage.

FIGS. 49A to 49C are diagrams showing the frequency distribution of themobility.

FIGS. 50A to 50C are diagrams showing the frequency distribution of thethreshold voltage.

FIGS. 51A to 51C are diagrams showing the frequency distribution of themobility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

An embodiment mode of the present invention is explained below withreference to the figures. A perspective view shown in FIG. 1 shows anembodiment in which a first insulating film 102, and second insulatingfilms 103 to 105 that are patterned into stripe shapes, are formed on asubstrate 101. Three stripe shape patterns are shown here for the secondinsulating films, but of course there are not limitations placed on thenumber of stripe shape patterns. The substrate can apply a commerciallyavailable non-alkaline glass substrate, a quartz substrate, a sapphiresubstrate, a single crystal or a polycrystalline semiconductor substratewhose surface is covered by an insulating film, or a metal substratewhose surface is covered by an insulating film.

The A width W1 of the second insulating films formed in stripe shapes isset from 0.1 to 10 μm (preferably from 0.5 to 1 μm), a gap W2 betweenadjacent second insulating films is set from 0.1 to 5 μm (preferablyfrom 0.5 to 1 μm), and a thickness d of the second insulating films isset to the same order as, or greater than, the thickness of a non-singlecrystal semiconductor film formed on the second insulating films.Further, step shapes need not have a regulated periodic pattern, and maybe formed to align with the layout and shape of island-like regionscontaining TFT channel formation regions. A length L of the secondinsulating films is also not limited, and the length L may be on anorder such that the TFT channel formation regions can be formed, forexample.

The first insulating film is formed using silicon nitride or siliconoxynitride. Further, the second insulating films are formed usingsilicon oxide or silicon oxynitride. Silicon oxide can be formed byplasma CVD by mixing tetraethyl-ortho-silicate (TEOS) and O₂—Siliconoxynitride films can be formed by plasma CVD using SiH₄, NH₃, and N₂O,or SiH₄ and N₂O as raw materials.

It is preferable to suitably regulate the materials and the filmformation conditions so that the etching speed of the second insulatingfilms becomes relatively fast in order to ensure selectivity duringetching processing in the case where an uneven shape is formed by thefirst insulating film and the second insulating films with openingportions as a form of FIG. 1. The angle of sidewalls of the openingportions formed of the second insulating film may be suitably set in arange from 5 to 120°, preferably in a range from 80 to 100°.

An amorphous semiconductor film 106 having a thickness of 50 to 200 nmis formed covering a surface composed of the first insulating film 102and the second insulating films 103 to 105, and the opening portions, asshown in FIG. 2. The amorphous semiconductor film can apply silicon, achemical compound or solid solution of silicon and germanium, or achemical compound or solid solution of silicon and carbon.

Continuous wave laser light is then irradiated to the amorphoussemiconductor film 106, performing crystallization. Applicable laserlight is concentrated and expanded into a linear shape by an opticalsystem. The intensity distribution of the laser light has a uniformregion in the longitudinal direction, and may possess a distribution inthe transverse direction. A rectangular shape beam solid state laseroscillation apparatus is applied as a laser oscillation apparatus whichis used as a light source, and in particular it is preferable to apply aslab laser oscillation apparatus. Alternatively, a solid state laseroscillation apparatus that uses a rod doped with Nd, Tm, and Ho, inparticular a solid state laser oscillation apparatus that uses a crystalsuch as YAG, YVO₄, YLF, YalO₃, or the like doped with Nd, Tm, and Ho mayalso be combined with a slap structure amplifier. The scanning is thenperformed as shown by an arrow in the figure in a direction thatintersects with the linear shape longitudinal direction. It is mostpreferable at this point to scan in a direction parallel to thelongitudinal direction of the stripe shape pattern formed in the baseinsulating film. Note that the term linear shape as used here denotes ashape in which the ratio of the length in the longitudinal direction,with respect to the length in the transverse direction, is equal to orgreater than 10.

Crystals such as Nd:YAG, Nd:GGG (gadolinium, gallium, garnet), Nd:GSGG(gadolinium, scandium, gallium, garnet) and the like can be used as theslab material. A zigzag light path is followed with the slab laser,while total reflection is repeated within the planer shape laser medium.

Further, considering the light absorption coefficient of the amorphoussemiconductor film, it is preferable that the wavelength of thecontinuous wave laser light be from 400 to 700 nm. Light in thiswavelength range is obtained by extracting the second harmonic or thethird harmonic of the fundamental wave using a wavelength converterelement. ADP (ammonium dihydrogen phosphate), Ba₂NaNb₅O₁₅ (barium sodiumniobate), CdSe (selenium cadmium), KDP (potassium dihydrogen phosphate),LiNbO₃ (lithium niobate), Se, Te, LBO, BBO, KB5, and the like can beapplied as the wavelength converter element. In particular, it ispreferable to use LBO. A typical example is to use the second harmonic(532 nm) of an Nd:YVO₄ laser oscillation apparatus (fundamental wave1064 nm). Further, the laser oscillation mode uses a single mode, whichis a TEM₀₀ mode.

For a case in which silicon is selected as the most suitable material,its absorption coefficient is in a region from 103 to 104 cm⁻¹, which isnear to the visible light region. If a substrate having a hightransmittivity of visible light is used, such as glass, and an amorphoussemiconductor film is formed of silicon having a thickness from 30 to200 nm, then crystallization can be performed without causing damage tothe base insulating film by irradiating visible light with a wavelengthof 400 to 700 nm, thus selectively heating the semiconductor region.Specifically, the penetration distance of 532 nm wavelength light isnearly 100 to 1000 nm with respect to an amorphous silicon film, andinside portions of the amorphous semiconductor film 106 formed at a filmthickness of 30 to 200 nm can be sufficiently reached. That is, it ispossible to heat from the inside of the semiconductor film, and almostthe entire semiconductor film can be heated uniformly in the laser lightirradiation region.

The semiconductor film that has been melted by the laser lightirradiation aggregates in the opening portions (depressions) due to theeffect of surface tension. The surface is nearly flat in a state thussolidified, as shown by FIG. 3. In addition, crystal growth ends,crystal grain boundaries, and crystal subboundaries are formed on thesecond insulating films (projections, regions 110 denoted by hatch marksin the figure). A crystalline semiconductor film 107 is thus formed.

The crystalline semiconductor film 107 is then etched, formingisland-like semiconductor regions 108 and 109 as shown in FIG. 4. Onlygood quality semiconductor regions can remain in accordance with etchingand removing the regions 110 in which the growth ends, crystal grainboundaries, and crystal subboundaries are concentrated. A gateinsulating film and gate electrodes are then formed so that channelformation regions can be positioned by using the crystallinesemiconductor film filling the opening portions (depressions). TFTs canthus be completed through each stage.

FIGS. 5A to 5E are diagrams for schematically explaining therelationship between the depth of, and gap between, opening portiongrooves (steps) formed by the first insulating film 102 and the secondinsulating films 103 to 105, and crystal growth. Note that referencesymbols relating to lengths shown in FIGS. 5A to 5E are as follows: t01denotes the thickness of the amorphous semiconductor film on the secondinsulating films (projections); t02 denotes the thickness of theamorphous semiconductor film of the opening portions (depressions); tilldenotes the thickness of the crystalline semiconductor film on thesecond insulating films (projections); t12 denotes the thickness of thecrystalline semiconductor film of the opening portions (depressions); ddenotes the thickness of the second insulating films (depth of theopening portions); W1 denotes the width of the second insulating films;and W2 denotes the width of the opening portions.

FIG. 5A is for a case in which d<t02, and W1, W2≦1 μm. If the depth ofthe opening portion grooves is less than the thickness of the amorphoussemiconductor film 106, the semiconductor does not fill the openingportions even if it passes through a melting and crystallizationprocess, and the surface of the crystalline semiconductor film does notbecome planarized. That is, the shape with depressions and projectionsof the base of the crystalline semiconductor film remains nearlypreserved.

FIG. 5B is for a case in which d≧t02, and W1, W2≦1 μm. If the depth ofthe opening portion grooves is nearly equal to, or greater than, thethickness of the amorphous semiconductor film 106, surface tensioneffects and the amorphous semiconductor aggregates in the openingportions (depressions). The surface becomes nearly flat as shown in FIG.5B with a state thus solidified. In this case till becomes less thant12, stress concentrates in portions 120 having a thin film thickness,distortions concentrate, and further, crystal growth ends are formedthere.

FIG. 5C is for a case in which d>>t02, and W1, W2≦1 μm. In this case thecrystalline semiconductor film 107 is formed so as to fill the openingportions, and almost none of the crystalline semiconductor film remainson the second insulating films.

FIG. 5D is for a case in which d≧t02, and W1, W2>1 μm. If the width ofthe opening portions is expanded, then although the crystallinesemiconductor film fills the opening portions and there is a planarizingeffect, crystal grain boundaries and crystal subboundaries develop nearthe center of the opening portions. Further, stress similarlyconcentrates on the second insulating films, distortions aggregate, andfurther, crystal growth ends are formed there. It is conjectured thatthis is due to the stress relief effect being reduced by the wide gaps.

FIG. 5E is for a case in which d≧t02, and W1, W2>>1 μm, and the state ofFIG. 5D is further realized.

As explained above using FIGS. 5A to 5E, the conditions of FIG. 5B canbe considered the most suitable when forming a semiconductor element,particularly when forming a TFT.

One example is shown in the above explanation for the depression andprojection shape of the base for forming the crystalline semiconductorfilm, formed by the first insulating film and the second insulatingfilms, but the embodiment mode shown here is not limited to this shape.Bases having similar shapes may also be used. For example, openingportions may be formed directly in the surface of a quartz substrate,thus forming an uneven shape.

FIGS. 6A and 6B show an example of a structure of a laser processingapparatus capable of being applied to crystallization. FIGS. 6A and 6Bare a diagram showing a front view and a side view of the structure of alaser processing apparatus made from a laser oscillation apparatus 301,a shutter 302, high conversion mirrors 303 to 306, a slit 307,cylindrical lenses 308 and 309, a holding platform 311, driving means312 and 313 for displacing the holding platform 311 in the x-directionand the y-direction, a controlling means 314 for controlling the drivingmeans, an information processing means 315 for sending signals to thelaser oscillation apparatus 301 and the controlling means 314 based on aprogram stored in advance, and the like.

Laser light concentrated into a linear shape in the cross sectionalshape of an irradiation surface by the cylindrical lenses 308 and 309 ismade incident at an incline to a surface of a substrate 320 on theholding platform 311. This is done so that a linear shape focused lightsurface can be formed in the irradiation surface or its vicinity whenthe focal point position shifts due to aberrations such as astigmatisms.A high transmittivity is obtained if the cylindrical lenses 308 and 309are manufactured from synthetic quartz, and a coating implemented to thesurfaces of the lenses is applied in order to achieve a transmittivityequal to or greater than 99% with respect to the wavelength of the laserlight. The cross sectional shape of the irradiation surface is of coursenot limited to a linear shape, and arbitrary shapes such as arectangular shape, an elliptical shape, and an oval shape may also beused. Whichever shape is used, shapes having a ratio between the minoraxis and the major axis contained in a range from 1 to 10, to 1 to 100,are indicated. Further, the wavelength converter element 310 is preparedin order to obtain a harmonic with respect to a fundamental wave.

As stated above, a rectangular beam solid state laser oscillationapparatus is applied to the laser oscillation apparatus, and inparticular, it is preferable to apply a slab laser oscillationapparatus. Alternatively, a solid state laser oscillation apparatus thatuses a crystal such as YAG, YVO₄, YLF, YAlO₃, or the like doped with Nd,Tm, and Ho may also be combined with a slab structure amplifier.Crystals such as Nd:YAG, Nd:GGG (gadolinium, gallium, garnet), Nd:GSGG(gadolinium, scandium, gallium, garnet) and the like can be used as theslab material. In addition, a gas laser oscillation apparatus, or asolid state laser oscillation apparatus, capable of continuousoscillation can also be applied. Laser oscillation apparatuses usingcrystals such as YAG, YVO₄, YLF, YAlO₃, or the like doped with Cr, Nd,Er, Ho, Ce, Co, Ti, or Tm are applied as continuous wave solid statelaser oscillation apparatuses. Although differing by the dopantmaterial, the fundamental wave is emitted at a wavelength from 1 μm to 2μm. A diode excited solid state laser oscillation apparatus may beapplied in order to obtain very high output, and a cascade connectionmay also be used.

Further, laser processing of the substrate 320 is possible by moving theholding platform 311 in two axial directions by using the driving means312 and 313. For movement in one direction, it is possible tocontinuously move the holding platform 311 at a constant velocity from 1to 200 cm/sec, preferably from 5 to 50 cm/sec, over a distance longerthan the length of one side of the substrate 320. It becomes possible toperform non-continuous stepwise movement in the other direction over adistance on the same order as that of the longitudinal direction of thelinear shape beam. Emission of the laser oscillation apparatus 301, andthe holding platform 311 are operated synchronously by the informationprocessing means 315 in which a microprocessor is mounted.

It is possible to process the entire surface of the substrate by laserlight irradiated from a fixed optical system by linear motion of theholding platform 311 in the x-direction shown in the figure. A positiondetecting means 316 detects that the substrate 320 is in a position forirradiating laser light, and transfers this signal to the informationprocessing means 315. Emission operations of the laser oscillator andtheir timing are synchronized by the information processing means 315.That is, the laser oscillation is stopped when the substrate 320 is notin a position for the irradiation of laser light, and its lifetime isextended.

Laser light emitted to the substrate 320 by a laser irradiationapparatus having this type of structure can process desired regions, orthe entire surface, of the semiconductor film by relative motion in thex-direction and the y-direction shown in the figure.

By forming the step shapes in the base insulating film, distortions andstress that accompany crystallization of those portions can beconcentrated during crystallizing in which continuous wave laser lightis irradiated to an amorphous semiconductor film, as stated above. Thedistortions and stress can be prevented from occurring in thecrystalline semiconductor film that is made into an active layer. Byforming TFTs so that channel formation regions are disposed in thecrystalline semiconductor film which is free of distortions and stress,it becomes possible to increase current driving performance at highspeed, and it also becomes possible to increase element reliability.

Further, a method of irradiating laser light used by the presentinvention is explained using FIGS. 20A to 20C.

First, a first base film 9101 made from an insulating film is formed ona substrate as shown in FIG. 20A. Second base films 9102 made fromrectangular shape insulating films are then formed on the first basefilm 9101, and a third base film 9103 is formed so as to cover the firstbase film 9101 and the second base films 9102. Silicon nitride is usedas the first base film 9101 in this embodiment mode, silicon oxide isused as the second base film 9102, and a silicon oxide film is used asthe third insulating film 9103. Note that the materials used in thefirst insulating film 9101, the second insulating films 9102, and thethird insulating film 9103 are not limited to the aforementionedmaterials. Other materials may also be used provided that: they areinsulating films capable of withstanding heat treatment performed inlater process steps; they can prevent the contamination of alkalinemetals, which can cause adverse influence to TFT characteristics, to asemiconductor film formed later; and depressions and projections can beformed on them. Note that a method for forming this depressions andprojections is explained in detail later. Further, insulating filmsother than these may also be used, and a laminated structure of two ormore films may also be used.

Although the first insulating film, the second insulating films, and thethird insulating film are shown separately in FIGS. 20A to 20C, thethree base films are all taken together and referred to as a base film9104. Note that, although the base film 9104 having depressions andprojections is formed by using the three base films in this embodimentmode, the structure of the base film used by the present invention isnot limited to this structure.

A marker may also be formed at the same time as the base film 9104 byutilizing a portion of the base film.

The substrate may be the one capable of withstanding the processingtemperatures of later process steps. For example, quartz substrates,silicon substrates, glass substrates such as barium borosilicate glassand aluminum borosilicate glass, and metal substrates and stainlesssteel substrates on whose surface an insulating film is formed can besued. Further, plastic substrates having thermal resistancecharacteristics to an extent capable of withstanding the processingtemperatures may also be used.

A semiconductor film 9105 is formed next so as to cover the base film9104. The semiconductor film 9105 can be formed by a known method (suchas sputtering, LPCVD, or plasma CVD). Note that the semiconductor filmmay be an amorphous semiconductor film, and may also be amicrocrystalline semiconductor film or a crystalline semiconductor film.Further, not only silicon, but also germanium may also be used.Furthermore, impurities can be prevented from contamination between thesemiconductor film and the base film by forming the semiconductor filmin succession, without exposure to the atmosphere, after forming thethird base film 9103.

Note that a designer can suitably determine the shape and the size ofprojections of the base film 9104. It is necessary, however, to set thethickness of the projections on an order such that the semiconductorfilm formed later is not cut off in the vicinity of the edges of theprojections.

Laser light is irradiated to the semiconductor film 9105 next as shownin FIG. 20B. The semiconductor film 9105 melts once due to the laserlight irradiation, and its volume moves from upper portions of theprojections toward the depressions, as shown by the arrows having awhite center. A semiconductor film (after LC) 9106 having a planarizedsurface and increased crystallinity is formed. The energy density of thelaser light becomes lower in the vicinity of the edges of the laserbeam, and therefore the crystal grains become smaller in the vicinity ofthe edges, and protruding portions (ridges) appear along the crystalgrain boundaries. Irradiation is therefore performed such that the edgesof the laser light beam path is not overlapped with portions that becomechannel formation regions and portions located on the depressions of thesemiconductor film 9105.

Note that the scanning direction for the laser light is set so that itbecomes parallel to the longitudinal direction of the projections of thebase film 9104.

Known lasers can be used by the present invention. it is preferable thatthe laser light be continuous wave, but it can be considered that acertain level of the effect of the present invention can be obtainedeven if a pulse wave type is used. The laser can use a gas laser or asolid state laser. Excimer lasers, Ar lasers, Kr lasers and the likeexist as gas lasers, and YAG lasers, YVO₄ lasers, YLF lasers, YAlO₃lasers, glass lasers, ruby lasers, Alexandrite lasers, Ti:sapphirelasers, Y₂O₃ lasers and the like can be given as solid state lasers.Lasers using crystals such as YAG, YVO₄, YLF, YAlO₃, or the like dopedwith Cr, Nd, Er, Ho, Ce, Co, Ti, Yb or Tm are applied as the solid statelaser. Although differing by the material used in doping, laser lighthaving a fundamental wave of approximately 1 μm is obtained. Harmonicswith respect to the fundamental wave can be obtained by using anon-linear optical element.

In addition, after converting infrared laser light emitted from a solidstate laser to green laser light by using a non-linear optical element,ultraviolet laser light then obtained by using another non-linearoptical element can also be used.

As for the semiconductor film (after LC) 9106, its film thicknessbecomes thicker on the depressions of the base film 9104, and converselyits film thickness becomes thinner on the projections of the base film9104, by volumetric movement due to the laser light irradiation. Grainboundaries 9149 therefore easily generate on the projections due tostress, and conversely, a state with nearly good crystallinity isobtained on the depressions. Note that it is not always true that thesemiconductor film (after LC) contains no grain boundaries on thedepressions. However, even if grain boundaries do exist, the crystalgrains are large, and therefore the crystallinity becomes relativelysuperior.

The surface of the semiconductor film (after LC) 9106 is then etched,thus exposing upper surfaces of the projections of the base film 9104.Note that the semiconductor film (after LC) 9106 is etched so as toexpose the upper surfaces of the projections of the base film 9104 inthis embodiment mode. Crystalline semiconductor films (islands) 9107 arethen formed in the depressions of the base film 9104 as shown in FIG.20C.

By using the islands obtained in accordance with the above series ofprocess steps as TFT active layers, more preferably as TFT channelformation regions, the formation of grain boundaries in the TFT channelformation regions can be prevented, and conspicuous drops in the TFTmobility, reductions in the ON current, and increases in the OFFcurrent, all due to grain boundaries, can be prevented. Note that adesigner can suitably determine a portion to be removed as being in thevicinity of the edges of the depressions, or the edges of theprojections, by patterning.

EXAMPLES

Examples of the present invention are explained below.

Example 1

This Example shows an example of manufacturing a TFT in which acrystalline silicon film is formed on a base insulating film having anopening portion, and a channel formation region is disposed in a filledregion filling the opening portion.

A first insulating film 602 is formed by silicon nitride film with athickness of a 100 nm on a glass substrate 601 in FIGS. 7A to 7C. Asilicon oxide film is formed on the first insulating film 602, andsecond insulating films 603 having a rectangular shape pattern areformed by photoetching. The silicon oxide film is deposited to athickness of 150 nm by using plasma CVD with a mixture of TEOS and O₂,and by discharging at a high frequency (13.56 MHz) electric powerdensity of 0.6 W/cm with a reaction pressure of 40 Pa and a substratetemperature of 400° C. Opening portions 604 a and 604 b are then formedby etching.

Note that FIG. 7A is an upper surface diagram, FIG. 7B is a longitudinalcross sectional diagram corresponding to a line segment A-A′ in FIG. 7A,and FIG. 7C is a longitudinal cross sectional diagram corresponding to aline segment B-B′ in FIG. 7A. FIGS. 8A to 8C, 9A to 9C, 10A to 10C, 11Ato 11C, and 12A to 12C are similarly handled.

An amorphous silicon film 605 covering the first insulating film 602 andthe second insulating films 603 is then formed at a thickness of 150 nmas shown in FIGS. 8A to 8C. The amorphous silicon film 605 is formed byusing plasma CVD with SiH₄ as a raw material gas.

The continuous wave laser light is then irradiated, thus performingcrystallization as shown in FIG. 9A to 9C. The crystallizationconditions are as follows: a continuous wave mode of a YVO₄ laseroscillator is used; a second harmonic (wavelength 532 nm) output of 5.5W is condensed by an optical system so as to have a uniform energydistribution in a longitudinal direction, with a size of 400 μm in thelongitudinal direction and from 50 to 100 μm in a transverse direction;and this is scanned at a velocity of 50 cm/sec, thus causingcrystallization. The term uniform energy density distribution does notimply excluding energy density distribution that is not completelyuniform, but denotes one with a permissible range for the energy densitydistribution of +20%. The laser processing apparatus structure as shownin FIGS. 6A and 6B can be applied to this type of laser lightirradiation. The laser light condensed by the optical system may have arange in the longitudinal direction wherein the intensity distributionis uniform, and may possess a distribution in the transverse direction.Crystallization is performed such that the intensity distribution formsa uniform region in the longitudinal direction, and the effectivenessfor crystal growth in a direction parallel to the laser light scanningdirection can be increased.

The amorphous silicon film is melted instantaneously by irradiating thelaser light under these conditions, and crystallization proceeds as themelted stripe moves. Surface tensions effects on the melted silicon, andthe melted silicon aggregates in the opening portions (depressions) andsolidifies. A crystalline semiconductor film 606 is thus formed in astate wherein it fills the opening portions 604 a and 604 b.

A mask pattern is then formed and an etching process is implemented sothat at least the crystalline semiconductor film remains in the openingportions 604 a and 604 b, thus forming island-like regions 607 and 608that contain channel formation regions as shown in FIGS. 10A to 10C.

FIGS. 11A to 11C show a state in which a gate insulating film 609, andgate electrodes 610 and 611 are formed on an upper layer side of thesemiconductor regions 607 and 608. The gate insulating film 609 may beformed from a silicon oxide film with a film thickness of 80 nm by usingplasma CVD. The gate electrodes 610 and 611 are formed by using tungstenor an alloy containing tungsten. Channel formation regions can be formedin the island-like semiconductor regions that fill the opening portions604 a and 604 b by employing this type of structure.

TFTs can then be completed, provided that source and drain regions, lowconcentration drain regions, and the like are suitably formed.

Example 2

Example 2 shows a structure formed by processes that are similar tothose of Example 1, but as shown by FIGS. 12A to 12C, the openingportions formed in the second insulating films 603 are formed in long,thin strip regions and regions that are connected to the long, thinstrip regions. As island-like semiconductor region 620 is formed by acrystalline silicon film in alignment with the opening portion 604 c,and a single gate, multi-channel TFT can be completed by forming a gateinsulating film 621 and a gate electrode 622.

Example 3

By forming the thickness of the second insulating films thicker than theamorphous semiconductor film in Example 2, for example by forming thesecond insulating films at 350 nm, the island-like region 620 formed bythe crystalline semiconductor film can be completely buried in anopening portion 604 d as shown in FIGS. 13A to 13C. A single gate,multi-channel TFT can then be formed provided that the gate insulatingfilm 621 and the gate electrode 622 are similarly formed.

Example 4

FIGS. 14A to 14D show another example of a single gate, multi-channelTFT. The first insulating film 602, the second insulating films 603, anisland-like semiconductor region 630, a gate insulating film 631, and agate electrode 632 are formed on the substrate 601 similar to thoseformed in Examples 1 to 3. A portion that differ in FIGS. 14A to 14C isan opening portion 604 e formed by the second insulating films 603, andin addition, the second insulating films are removed from the peripheryof the semiconductor regions, wherein channel formation regions areformed, after forming the island-like semiconductor regions 630, thusforming second opening portions 625.

An enlarged view of an Example of the vicinity of the channel formationregion is shown in FIG. 14D. The gate electrode 631 is formed contactinga side surface and an upper surface of the island-like semiconductorregion 630, and the gate electrode 632 is formed covering the gateinsulating film 631. In this case the channel formation region is formedon an upper portion 634 and a side surface portion 635 of thesemiconductor region 630. Depleted regions can thus be increased, andthe TFT current driving performance can be improved.

Example 5

The present invention can be applied to various types of semiconductordevices, and an Example of a display panel manufactured based onExamples 1 to 4 is explained.

A pixel portion 902, gate signal line driver circuits 901 a and 901 b, asource signal line driver circuit 901 c, input terminals 935, andwirings or a wiring group 917 are prepared on a substrate 900 in FIGS.15A to 15C. A seal pattern 940 may be overlapped with a portion of thegate signal line driver circuits 901 a and 901 b, the source signal linedriver circuit 901 c, and the wirings or wiring group 917 that connectsdriver circuit portion with the input terminals 935. The surface area ofa frame region (peripheral region of the pixel portion) of the displaypanel can thus be reduced. An FPC 936 is fixed to the input terminals935.

In addition, a chip 950 on which a microprocessor, memory, or mediaprocessor/DSP (digital signal processor) or the like is formed usingTFTs of the present invention may also be mounted. These functionalcircuits are formed by using design rules different from those of thepixel portion 902, the gate signal line driver circuits 901 a and 901 b,and the source signal line driver circuit 901 c. Specifically, a designrule of 1 μm or less is applied. There are no limitations placed on themounting method, and a COG method or the like may be applied.

The TFTs shown by Examples 1 to 4 can be applied as switching elementsof the pixel portion 902, and in addition, as functional elements thatstructure the gate signal line driver circuits 901 a and 901 b and thesource signal line driver circuit 901 c.

FIG. 16 is an example showing the structure of one pixel of the pixelportion 902, and TFTs 801 to 803 are prepared. These TFTs are switchingTFTs, reset TFTs, and driver TFTs for controlling light emittingelements or liquid crystal elements prepared in the pixels.

Island-like semiconductor regions 812 to 814 containing channelformation regions of these TFTs are formed in alignment with openings809 to 811 in a base insulating film formed in a layer below thesemiconductor regions. The island-like semiconductor regions 812 to 814can be formed based on Examples 1 to 5. Gate wirings 815 to 817 areformed over the island-like semiconductor regions 812 to 814, and asignal line 818, an electric power line 819, other types of wirings 820and 821, and a pixel electrode 823 are formed through a passivation filmand a planarizeing film.

The present invention can thus complete the display panel withoutcausing any influence on the display panel.

Example 6

A process of manufacturing a so-called multi-channel TFT, which has aplurality of channel formation regions that are mutually separated fromeach other, by using the semiconductor device of the present invention,is discussed in Example 6.

First, a base film 9120 having projections is formed on an insulatingsurface as shown in FIG. 21A. Note that a cross sectional diagram takenalong a line segment A-A′ of FIG. 21A is shown in FIG. 21B, and a crosssectional diagram taken along a line segment B-B′ of FIG. 21A is shownin FIG. 21C.

The base film 9120 used in Example 6 has the same structure as thatshown in the Embodiment Mode. The base film 9120 is composed of threebase films. First, a second base film 9122 made from rectangular shapesilicon oxide is formed on a first base film 9121 made from siliconnitride, and a third base film 9123 made from silicon oxide is formed soas to cover the first base film 9121 and the second base film 9122. Thebase film 9120 is formed by the first base film 9121, the second basefilm 9122, and the third base film 9123 in Example 6. Projections 9124of the base film 9120 are structured by the rectangular shape secondinsulating film 9122 and by portions of the third insulating film 9123that contact the second insulating film 9122 but do not contact thefirst insulating film 9121.

Note that, although a designer can suitably set the shapes and the sizesof the depressions 9124, it is necessary to set the thickness on anorder such that cutoff of a later formed semiconductor film does notoccur in the vicinity of edges of the projections 9124. The height ofthe depressions 9124 is set on the order of 0.1 to 1 μm in Example 6.

Note that distortions of the substrate cause influence to the shape ofthe base film later formed. Distortions of the base film become a causeof disturbances in the uniformity of the crystallinity of thesemiconductor film formed later, and therefore the surface of thesubstrate may be polished using a chemical mechanical polishing method(CMP method) so that the distortion difference is suppressed to a levelequal to or less than 10 nm, and the substrate may be heated in advanceprior to forming the base film so that the substrate does not distortdue to heat treatment processing in later process steps.

An amorphous semiconductor film 9125 is then formed covering the basefilm 9120. The non-single crystalline semiconductor film 9125 can beformed by using a known method (such as sputtering, LPCVD, or plasmaCVD). The non-single crystalline semiconductor film 9125 is formedhaving a thickness of 300 nm by plasma CVD in Example 6.

Laser light is next irradiated to the non-single crystal semiconductorfilm 9125 as shown in FIG. 22A, thus performing crystallization. Notethat FIG. 22B corresponds to a cross sectional diagram taken along adashed line segment A-A′ of FIG. 22A. Irradiation is performed inExample 6 by using a continuous wave YVO₄ laser at a scanning speed of50 cm/sec. The laser scanning direction at this point is such that it isaligned with the same direction as the carrier moves in channelformation regions formed later. Laser light is irradiated while thescanning direction is aligned with the longitudinal direction of therectangular shape projections 9124 in Example 6, as shown by the arrowhaving a white center. The non-single crystal semiconductor film 9125melts due to the irradiation of laser light, and its volume moves fromthe projections 9124 to the depressions, thus forming a crystallinesemiconductor film 9126.

The crystalline semiconductor film 9126 is patterned next as shown inFIG. 23A, thus forming a sub-island 9127. Note that FIG. 23B correspondsto a cross sectional diagram of FIG. 23A along a dashed line segmentA-A′. Portions of the sub-island 9127 exist on the projections formedbetween the depressions 9124. The channel formation regions of theobjective multi-channel TFTs are formed using portions of thecrystalline semiconductor film 9126 located on the depressions, andtherefore it is essential that the positional relationship between thesub-island 9127 and the projections 9124 be determined by consideringthe number of channel formation regions, the channel length, and thechannel width.

An island 9128 is formed next by removing the sub-island 9127 from anupper surface to an extent such that upper surfaces of the projections9124 are exposed, as shown in FIG. 24A. Note that FIG. 24B correspondsto a cross sectional diagram taken along a dashed line segment A-A′ ofFIG. 24A. Removal of the sub-island 9127 from its upper surface may beperformed by using any method, for example the removal may be performedby using etching, and may also be performed by using a CMP method.

Portions in which grain boundaries exist on the projections 9124 areremoved by performing removal from the upper surface of the sub-island9127. Almost no grain boundaries exist on the depressions correspondingto spaces between the projections 9124, and crystalline semiconductorfilms having superior crystallinity remain in portions that later becomechannel formation regions. The slit shape island 9128 is in which onlychannel formation regions are separated is formed as shown in FIGS. 24Aand 24B. Note that portions that become source regions or drain regionsdo not influence the TFT characteristics, due to the semiconductor filmcrystallinity, as much as the channel formation regions do. The portionsthat become source regions or drain regions therefore do not become muchof a problem, even if their crystallinity is not good compared to thecrystallinity of the portions that become the channel formation regions.

A TFT is manufactured next by using the island 9128, as shown in FIG.25A. Note that there are various TFT structures and method ofmanufacturing TFTs. FIG. 25B corresponds to a cross sectional diagramtaken along a dashed line segment A-A′ of FIG. 25A, and FIG. 25Ccorresponds to a cross sectional diagram taken along a dashed linesegment B-B′ of FIG. 25A. FIG. 26A corresponds to a cross sectionaldiagram taken along a dashed line segment C-C′ of FIG. 25A, and FIG. 26Bcorresponds to a cross sectional diagram taken along a dashed linesegment D-D′ of FIG. 25A.

A channel formation region 9130 in the island 9128 is overlapped with agate electrode 9132, sandwiching a gate insulating film 9131therebetween. Further, the channel formation region 9130 is similarlysandwiched by two impurity regions 9133 of the island 9128. Note thatthe two impurity regions 9133 function as a source region or a drainregion.

A first interlayer insulating film 9134 is then formed covering theisland 9128, the gate insulating film 9131, and the gate electrode 9132.A second interlayer insulating film 9135 is then formed covering thefirst interlayer insulating film 9134. Note that the first interlayerinsulating film 9134 is an inorganic insulating film, and thatimpurities such as carbon in the second interlayer insulating film 9135can be prevented from entering the island 9128. Further, the secondinterlayer insulating film 9135 is an organic resin film, and has aneffect for planarizing the surface so that wirings formed later do notbecome cut.

A wiring 9136 that is connected to the impurity region 9133 through acontact hole formed in the gate electrode 9131, the first interlayerinsulating film 9134, and the second interlayer insulating film 9135 isthen formed on the second interlayer insulating film 9135.

A TFT having a plurality of mutually separated channel formation regionsis thus completed by the above manufacturing processes. Heat generatedby driving the TFT can be effectively dissipated by using this type ofstructure.

Note that the TFT structure is not limited to that shown in FIGS. 25A to25C in the present invention. Further, the number of channel formationregions is not limited to four, and the number of channel formationregions present may be one, four, or other numbers than four.

Further, the TFT structure is not limited to the aforementionedstructure. For example, the TFT structure may have a structure like thatshown in FIGS. 27A and 27B. The TFTs shown in FIG. 27A have gateelectrodes composed of two layer conductive films 9140 and 9141.Sidewalls 9142 made from an insulating film are formed contacting uppersurfaces of the conductive films 9140 and side surfaces of theconductive films 9141. For example, TaN can be used as the conductivefilms 9140, W can be used as the conductive films 9141, and SiO₂ or thelike can be used as the sidewalls 9142. The TFTs shown in FIG. 28B havegate electrodes composed of two layer conductive films 9144 and 9145.The conductive films 9144 are overlapped with portions of impurityregions.

Note that stress which develops within the semiconductor film can berelieved by performing heat treatment at 500 to 600° C. for a period onthe order of one minute to 60 minutes, after laser light irradiation orafter etching the crystalline semiconductor film to an extent such thatthe projections 9124 of the base film are exposed.

The formation of grain boundaries in the TFT channel formation regionscan be prevented by actively using the semiconductor films located onthe depressions of the insulating film as TFT active layers, andconspicuous drops in the TFT mobility, reductions in the ON current, andincreases in the OFF current, all due to grain boundaries, can all beprevented.

Example 7

A method of manufacturing an island in which the process order differsfrom that of Example 6 is explained in Example 7. Note that Example 6may be referred to for detailed explanations of each process step.

First, a base film having rectangular shape projections 9301 is formedas shown in FIG. 28A, and a non-single crystal semiconductor film 9302is formed on the base film. Laser light is next irradiated to thenon-single crystal semiconductor film 9302, thus forming a crystallinesemiconductor film 9303 (FIG. 28B).

Next, a portion of the crystalline semiconductor film 9303 is removedfrom its surface to an extent such that upper surfaces of theprojections 9301 are exposed. Note that removal is performed usingetching in Example 7, and the crystalline semiconductor film afterremoval becomes a crystalline semiconductor film (after etching) 9304(FIG. 28C).

The crystalline semiconductor film (after etching) 9304 is patternednext, thus forming an island 9305 (FIG. 28D).

Note that stress which generates within the semiconductor film can berelieved by performing heat treatment at 500 to 600° C. for a period onthe order of one minute to 60 minutes, after etching the crystallinesemiconductor film to an extent such that the projections of the basefilm are exposed, or after forming the island.

Removal of edge portions and side portions of the island due to etchingcan be prevented in accordance with the above process steps by etchingthe crystalline semiconductor film to an extent such that theprojections of the base film are exposed before forming the island.

Example 8

A method of manufacturing an island in which the process order differsfrom that of Example 6 and Example 7 is explained in Example 8. Notethat Example 6 may be referred to for a detailed explanation of eachprocess step.

First, a base film having projections with rectangular shape 9311 isformed as shown in FIG. 29A, and a non-single crystal semiconductor film9312 is formed on the base film.

The non-single crystal semiconductor film 9312 is patterned next, thusforming a sub-island 9313 (FIG. 29B).

Laser light is irradiated to the sub-island 9313 next, thus causingcrystallization. The sub-island after crystallization becomes asub-island (after crystallization) 9314 in Example 8 (FIG. 29C).

Portions of the sub-island (after crystallization) 9314 are removed nextfrom its surface, to an extent such that upper surfaces of theprojections 9311 are exposed. Note that the removal is performed byetching in Example 8, thus forming an island 9315 (FIG. 29D).

Note that stress which develops within the semiconductor film can berelieved in accordance with the above process steps by performing heattreatment at 500 to 600° C. for a period on the order of one minute to60 minutes, after laser light irradiation or after forming the island.

Example 9

An example of forming a multi-channel TFT and a single channel TFThaving only one channel formation region by using a plurality ofprojections is explained in Example 9.

A base film having a plurality of rectangular shape projections 9330 isshown in FIG. 30A. TsFT that use islands formed on the base film areshown in FIG. 30B. FIG. 30B has a multi-channel TFT 9331 having fourchannel formation regions, a multi-channel TFT 9332 having two channelformation regions, and a single channel TFT 9333.

Each TFT is formed on a depressions located between the projections9330. It is more preferable that the channel formation regions and LDDregions be formed on the depressions located between the projections9330.

It is possible to implement Example 9 in combination with Examples 6 to8.

Example 10

An example of combining a process of irradiating laser light with aprocess of crystallizing a semiconductor film using a catalyst whencrystallizing the semiconductor film is explained in Example 10. It ispreferable to use the techniques disclosed by JP 7-130652 A and JP8-78329 A when using a catalyst element.

First, a non-single crystal semiconductor film 9352 is formed on a basefilm 9351 having projections 9350 as shown in FIG. 31A. Next, thenon-single crystal semiconductor film 9352 is crystallized by using acatalytic element (FIG. 31B). For example, if the technique disclosed inJP 7-130652 A is used, then a nickel acetate solution containing nickelof 10 ppm is applied to the non-single crystal semiconductor film 9352,thus forming a nickel containing layer 9353. A dehydrogenation processis performed for one hour at 500° C., after which heat treatment isperformed at 500 to 650° C. for 4 to 12 hours, for example at 550° C.for 8 hours, thus forming a crystalline semiconductor film 9354 havingimproved crystallinity. Note that, in addition to nickel (Ni), catalyticelement capable of being used include the elements germanium (Ge), iron(Fe), palladium (Pd), tin (Sn), lead (Pb) cobalt (Co), platinum (Pt),copper (Cu), and gold (Au).

A crystalline semiconductor film (after LC) 9355 having additionallyimproved crystallinity is then formed by laser irradiation from thecrystalline semiconductor film (after NiSPC) 9354 crystallized by NiSPC(FIG. 31C). The crystalline semiconductor film (after LC) 9355 meltsonce during laser light irradiation, and moves volumetrically from upperportions of the projections 9350 toward depressions, thus planarizingits surface. The film thickness on the projections 9350 becomes thin,and grain boundaries 9356 easily generate due to stress.

A process for gettering the catalytic element within the crystallinesemiconductor film (after LC) 9355 is explained next. Note that,although gettering is performed after laser light irradiation in Example10, it may also be performed after etching the crystalline semiconductorfilm (after LC) 9355.

A barrier layer 9357 having silicon as its main constituent is thenformed on the crystalline semiconductor film (after LC) 9355 (FIG. 31D).Note that the barrier layer 9357 may be extremely thin, may be a filmoxide naturally, and may be an oxide film oxidized by generating ozoneby ultraviolet light irradiation under an atmosphere containing oxygen.Further, an oxide film oxidized by a liquid containing ozone and usedduring surface processing referred to as hydro-washing and performed inorder to remove carbon, that is organics, may also be used. The barrierlayer 9357 is mainly used as an etching stopper. Further, channel dopingmay be performed after forming the barrier layer 9357, after whichactivation may be performed by irradiating strong light.

A first semiconductor film 9358 used in gettering is next formed on thebarrier layer 9357. The first semiconductor film 9358 used for getteringmay be a semiconductor film having an amorphous structure, and may alsobe a semiconductor film having a crystalline structure. The filmthickness of the first semiconductor film 9358 used for gettering is setfrom 5 to 50 nm, preferably from 10 to 20 nm. It is preferable toincrease the gettering efficiency by including oxygen in the firstsemiconductor film 9358 used for gettering (at a SIMS analysisconcentration equal to or greater than 5×10¹⁸ atoms/cm³, preferablyequal to or greater than 1×10¹⁹ atoms/cm³).

A third semiconductor film (gettering site) 9359 containing an inert gaselement is formed next to the first semiconductor film 9358 used forgettering. The second semiconductor film 9359 used for gettering may bea semiconductor film having an amorphous structure, or a semiconductorfilm having a crystal structure, and may be formed by using plasma CVD,low pressure thermal CVD, or sputtering. The second semiconductor filmmay be a semiconductor film containing an inert gas element at the filmformation stage, and an inert gas element may also be added after filmformation of a semiconductor film that does not contain a inert gaselement. The example shown in Example 10 is one in which the secondsemiconductor film 9359 used for gettering and containing an inert gaselement at the film formation stage is formed, after which an additionalinert gas element is selectively added, thus forming the secondsemiconductor film 9359 used for gettering. Further, the firstsemiconductor film and the second semiconductor film used for getteringmay be formed in succession without exposure to the outside atmosphere.Furthermore, the sum of the film thickness of the first semiconductorfilm and the film thickness of the second semiconductor film may be setfrom 30 to 200 nm, for example 50 nm.

A gap is prepared between the crystalline semiconductor film (after LC)9355 and the second semiconductor film 9359 by the first semiconductorfilm 9358 used for gettering. Impurity elements such as metals existingwithin the crystalline semiconductor film (after LC) 9355 tend to easilyaggregate in the vicinity of gettering site boundaries during gettering,and therefore it is preferable to increase efficiency by placing thegettering boundaries far from the crystalline semiconductor film (afterLC) 9355 by employing the first semiconductor film 9358 used forgettering as in Example 10. In addition, the first semiconductor film9358 used for gettering also has a blocking effect so that the impurityelements contained in the gettering site do not diffuse and reach theinterface with the first semiconductor film. Further, the firstsemiconductor film 9358 used in gettering also has a protective effectso that damage is not caused to the crystalline semiconductor film(after LC) 9355 in the case where an inert gas element is added.

Gettering is performed next. Heat treatment may be performed within anitrogen atmosphere at a temperature of 450 to 800° C. for one to 24hours, for example at 550° C. for 14 hours, as a process for performinggettering. Further, intense light may also be irradiated instead of heattreatment. Furthermore, a heated gas may be injected so as to heat thesubstrate. In this case heating may be performed for 1 to 60 minutes ata temperature from 600° C. to 800° C., more preferably from 650° C. to750° C., thus reducing the processing time. Impurity elements thus movein the second semiconductor film 9359 as shown by the arrows in FIG. 31Dby this gettering. Removal of the impurity elements, or a reduction inconcentration of the impurity elements, contained in the crystallinesemiconductor film (after LC) 9355 that is covered by the barrier layer9357 is thus performed. A crystalline semiconductor film (aftergettering) 9360 that contains almost no impurity elements, wherein theimpurity element concentration is equal to or less than 1×10¹⁸atoms/cm³, preferably equal to or less than 1×10¹⁷ atoms/cm³, is formedby gettering.

Next, the first semiconductor film 9358 used for gettering and thesecond semiconductor film 9359 are selectively removed with the barrierlayer 9357 used as an etching stopper.

After then changing the etching conditions and removing the barrierlayer 9357, as shown in FIG. 31E, the crystalline semiconductor film(after gettering) 9360 is etched to an extent such that upper surfacesof the projections 9350 are exposed, thus forming crystallinesemiconductor films 9361 in the depressions.

Note that crystal growth may also be performed by irradiating laserlight, not SPC, after applying a solution containing a catalytic elementto the semiconductor film before crystallization. Further, gettering mayalso use the techniques recorded in JP 10-135468, JP 10-135469, or thelike.

Note that although gettering is performed after laser light irradiationin Example 10, the present invention is not limited to this structure.Gettering may also be done after performing the etching of FIG. 31E.

It is possible to implement Example 10 in combination with Examples 6 to9.

Example 11

The structure of a laser irradiation apparatus used in the presentinvention is explained next using FIG. 32. Reference numeral 9151denotes laser oscillation apparatuses. Four laser oscillationapparatuses are used in FIG. 32, but the laser oscillation apparatusesof laser irradiation apparatus is not limited to this number.

Note that the laser oscillation apparatuses 9151 may be kept at aconstant temperature by using a chiller 9152. It is not always necessaryto use the chiller 9152, but dispersions in the energy of the outputlaser light due to temperature can be suppressed by maintaining thelaser oscillation apparatuses 9151 at a constant temperature.

Further, reference numeral 9154 denotes an optical system, and laserlight can be condensed by changing the path of the light output from thelaser oscillation apparatuses 9151 and by processing the shape of thelaser beam. In addition, the optical system 9154 in the laserirradiation apparatus of FIG. 32 can also synthesize the laser beams oflaser light output from the plurality of laser oscillation apparatuses9151 by making portions of the laser beams overlapped with each other.

Note that an AO modulator 9153 which changes the direction of advance ofthe laser light in an extremely short period of time may also be formedin the light path between a substrate 9156 to be processed and the laseroscillation apparatuses 9151. Further, an attenuator (filter forregulating the amount of light) may be formed as a substitute for the AOmodulator 9153, and the energy density of the laser light may beregulated.

Further, a measuring means (energy density measuring means) 9165 formeasuring the laser light energy density output from the laseroscillation apparatuses 9151 may be placed in the light path between thesubstrate 9156 to be processed and the laser oscillation apparatuses9151. Variations over time of the measured energy density may beobserved in a computer 9160. In this case, the output from the laseroscillation apparatuses 9151 may also be increased so as to compensatefor attenuation in the energy density of the laser light.

The synthesized laser beam is irradiated to the substrate 9156 to beprocessed through a slit 9155. It is preferable that the slit 9155 becapable of blocking laser light, and that it be formed by a substancethat is not changed or damaged by the laser light. The slit width of theslit 9155 is variable, and the width of the laser beam can be changed bythe slit width.

Note that the shape of the laser beam of laser light emitted from thelaser oscillation apparatuses 9151 not through the slit 9155 on thesubstrate 9156 differs by the laser type, and further, can be formed bythe optical system.

The substrate 9156 is placed on a stage 9157. Position controlling means9158 and 9159 correspond to means for controlling the position of thelaser beam on the piece to be processed, and the position of the stage9157 is controlled by the position controlling means 9158 and 9159 inFIG. 32.

The position controlling means 9158 performs control of the position ofthe stage 9157 in the x-direction in FIG. 32, and the positioncontrolling means 9159 performs position control of the stage 9157 inthe y-direction.

Further, the laser irradiation apparatus of FIG. 32 has the computer9160 prepared with both a storing means, such as memory, and a centraloperational processing apparatus. The computer 9160 controls theemission of the laser oscillation apparatuses 9151, determines thescanning path of the laser light, and moreover can control the positioncontrolling means 9158 and 9159 so that the laser beam of laser light isscanned along a determined path to move the substrate to thepredetermined position.

Note that, although the position of the laser beam in FIG. 32 iscontrolled by moving the substrate, it may also be moved by using anoptical system such as a galvano mirror, and both methods may also beused.

In addition, the width of the slit 9155 is controlled by the computer9160 in FIG. 32, but the width of the laser beam may also be changed inaccordance with mask pattern information. Note that it is not alwaysnecessary to form the slit.

In addition, the laser irradiation apparatus may also be provided with ameans for regulating the temperature of the piece to be processed.Further, the laser light is light having directivity and a high energydensity, and therefore a damper may be formed so that reflected light isprevented from being irradiated to inappropriate locations. It ispreferable that the damper have reflected light absorbing qualities, andchilled water may be circulated within the damper in order to preventthe barrier temperature from rising due to absorption of reflectedlight. Further, a means for heating the substrate (substrate heatingmeans) may also be formed on the stage 9157.

Note that a marker laser oscillation apparatus may also be provided inthe case where a marker is formed by laser. In this case, the emissionof the marker laser oscillation apparatus may be controlled by thecomputer 9160. In addition, an optical system for condensing the laserlight output from the marker laser oscillation apparatus may also bespecially provided in the case where the marker laser oscillationapparatus is used. Note that YAG lasers, CO₂ lasers, and the like can begiven as typical lasers used when forming the marker, and naturally itis possible to form the marker by using other lasers.

Further, one CCD camera 9163, or a plurality of the CCD cameras 9163depending upon the circumstances, may also be formed for positionalignment using the marker. Note that the term CCD camera denotes acamera that uses a CCD (charge coupled element) as a photographicelement.

Note that an insulating film pattern or a semiconductor film pattern mayalso be recognized by the CCD camera 9163, and position alignment forthe substrate may be performed, without forming the marker. In thiscase, insulating film or semiconductor film pattern information due to amask and input to the computer 9160 is compared to actual insulatingfilm or semiconductor film pattern information collected by the CCDcamera 9163, and in formation on the position of the substrate can beascertained. It is not necessary to specially form the marker in thiscase.

Further, laser light incident on the substrate is reflected by thesurface of the substrate, and returned along the same path as when beingincident. Thus so-called returned light exerts adverse influences suchas changing the laser output and its frequency, and destroying rods. Anisolator may therefore be disposed in order to remove the returned lightand stabilize the laser emission.

Note that although a structure in which a plurality of laser oscillationapparatuses are provided is shown in FIG. 32, one laser oscillationapparatus may also be used. The structure of a laser irradiationapparatus having one laser oscillation apparatus is shown in FIG. 33.Reference numeral 9201 in FIG. 33 denotes a laser oscillation apparatus,and reference numeral 9202 denotes a chiller. Further, reference numeral9215 denotes an energy density measuring means, reference numeral 9203denotes an AO modulator, reference numeral 9204 denotes an opticalsystem, reference numeral 9205 denotes a slit, and reference numeral9213 denotes a CCD camera. A substrate 9206 is disposed on a stage 9207,and the position of the stage is controlled by an x-direction positioncontrolling means 9208 and a y-direction position controlling means9209. Operation of each means of the laser irradiation apparatus is thencontrolled by a computer 9210, similarly to the structure shown in FIG.32. Differing from FIG. 32 is that there is one laser oscillationapparatus. Further, the optical system 9204 may have a function ofcondensing the one laser light, differing from the case of FIG. 32.

Note that the laser light is not scanned and irradiated over the entiresemiconductor film, but the laser light is scanned so that the minimumamount of crystallization of, at minimum, indispensable portions isperformed. Time for irradiating the laser light to portions that areremoved by patterning after crystallizing the semiconductor film can beomitted, and the amount of processing time per single substrates can begreatly reduced.

It is possible to implement Example 11 in combination with Examples 6 to10.

Example 12

A method of forming a base film having depressions and projections isexplained in Example 12.

At first, a first base film 9251 is formed from an insulating film on asubstrate 9250 as shown in FIG. 34A. The first base film 9251 usessilicon oxynitride in Example 12 but is not limited to this material,and insulating films having a large selectivity in etching with a secondbase film may be used. The first base film 9251 is formed by a CVDapparatus using SiH₄ and N₂O so that its thickness is from 50 to 200 nm.Note that the first base film may be a single layer, and may also be alaminated structure of a plurality of insulating films.

A second base film 9252 is formed next from an insulating material incontact with the first base film 9251, as shown in FIG. 34B. It isnecessary that the film thickness of the second base film 9252 be of anorder such that depressions and projections appear in the surface of asemiconductor film formed subsequently when patterning is performed in alater step, forming depressions and projections. A 30 nm to 300 nmsilicon oxide film is formed as the second base film 9252 by usingplasma CVD in Example 12.

A mask 9253 is formed next as shown in FIG. 34C, and the second basefilm 9252 is etched. Note that wet etching is performed at 20° C. inExample 12, using a mixed solution containing 7.13% ammonium hydrogenfluoride (NH₄HF₂) and 15.4% ammonium fluoride (NH₄F) (Stella ChemifaCorporation, product name LAL500) as an etchant. Projections withrectangular shape 9254 are formed by this etching. The first base film9251 and the projections 9254 are taken as one base film in thisspecification.

Note that it is preferable to pattern the second base film 9252 by usingRF sputtering in the case where aluminum nitride, aluminum oxynitride,or silicon nitride is used as the first base film 9251 and a siliconoxide film is used as the second base film 9252. The thermalconductivity of aluminum nitride, aluminum oxynitride, and siliconnitride as the first base film 9251 is high, and therefore generatedheat can quickly diffuse, and TFT deterioration can be prevented.

A semiconductor film is formed next so as to cover the first base film9251 and the projections 9254. The thickness of the projections is from30 nm to 300 nm in Example 12, and therefore it is preferable to set thefilm thickness of the semiconductor film from 50 to 200 nm, and it isset to 60 nm here. Note that adverse influences are caused to thesemiconductor film crystallinity if impurities are contaminated betweenthe semiconductor film and the base film. There is a possibility thatvariations in the characteristics of the manufactured TFTs, and thatvariations in the threshold voltages may increase, and therefore it ispreferable to form the base film and the semiconductor film insuccession. A silicon oxide film 9255 is formed thinly on the base filmin Example 12 after forming the first base film 9251 and the projections9254, and the semiconductor film 9256 is then formed without exposure tothe outside atmosphere. The thickness of the silicon oxide film can besuitably set by a designer, and is set on the order of 5 nm to 30 nm inExample 12.

A method of forming a base film that differs from that of FIG. 34 isexplained next. A first insulating film 9261 made from an insulatingfilm is first formed on a substrate 9260 as shown in FIG. 35A. The firstbase film is formed by a silicon oxide film, a silicon nitride film, asilicon oxynitride film, or the like.

If a silicon oxide film is used, it can be formed by plasma CVD bymixing tetraethyl orthosilicate (TEOS) and O₂, at a reaction pressure of40 Pa, at a substrate temperature of 300 to 400° C., and by dischargingat a high frequency (13.56 MHz) electric power density of 0.5 to 0.8W/cm². If a silicon oxynitride film is used, it may be formed by asilicon oxynitride film manufactured by plasma CVD from SiH₄, N₂O, andNH₃, or by a silicon oxynitride film manufactured by plasma CVD fromSiH₄ and N₂O. The manufacturing conditions in this case are thatformation can occur at a reaction pressure of 20 to 200 Pa, a substratetemperature of 300 to 400° C., and a high frequency (60 MHz) electricpower density of 0.1 to 1.0 W/cm². Further, a hydrogen siliconoxynitride film manufactured from SiH₄, N₂O, and H₂ may also be applied.It is possible to manufacture silicon nitride films similarly by plasmaCVD using SiH₄ and NH₃.

After forming the first base film over the entire substrate to have athickness of 20 to 200 nm (preferably between 30 and 60 nm), a mask 9262is then formed as shown in FIG. 35B by using a photolithographytechnique. Unnecessary portions are then removed by etching, thusforming rectangular shape projections 9263. A dry etching method thatuses a fluoride gas with respect to the first base film 9261 may beused, and a wet etching method that uses an aqueous solution of afluoride may be used. In the case where the latter method is selected,etching may be performed by a mixed solution (Stella ChemifaCorporation, product name LAL500) containing 7.13% ammonium hydrogenfluoride (NH₄HF₂) and 15.4% ammonium fluoride (NH₄F).

A second base film 9264 made from an insulating film is formed next soas to cover the projections 9263 and the substrate 9260. This film isformed by using a silicon oxide film, a silicon nitride film, a siliconoxynitride film, or the like at a thickness from 50 to 300 nm(preferably from 100 to 200 nm), similar to the first base film 9261.

A base film composed of the projections 9263 and the second base film9264 is formed in accordance with the above manufacturing processes.Note that impurities can be prevented from contamination between thesemiconductor film and the base film by forming the semiconductor filmin succession, without exposure to the atmosphere, after forming thesecond base film 9264.

It is possible to implement Example 12 by being freely combined withExamples 6 to 11.

Example 13

The shape of a laser beam synthesized by mutual overlap of varying laserbeams is explained in Example 13.

An example of the shape of a laser beam on a processing piece, for acase in which laser light emitted from a plurality of laser oscillationapparatuses does not pass through a slit, is shown in FIG. 36A. Thelaser beams shown in FIG. 36A have elliptical shapes. Note that theshape of the laser beam of laser light emitted from a laser oscillationapparatus in the present invention is not limited to an ellipticalshape. The shape of the laser beam differs depending on the type oflaser used, and further, can be shaped by an optical system. Forexample, the shape of laser light emitted from a Lambda Corporation XeClexcimer laser (wavelength 308 nm, pulse width 30 ns) L3308 is a 10 mm×30mm (half-value widths in the beam profile) rectangular shape. Further,the shape of laser light emitted from a YAG laser becomes circular ifthe rod shape is cylindrical, and becomes rectangular if a slab is used.Laser light having a desired size can be made by performing additionalshaping of this type of laser light with an optical system.

FIG. 36B shows the distribution of the energy density of laser light inthe longitudinal axis Y direction of the laser beam shown in FIG. 36A.The laser beam shown in FIG. 36A corresponds to a region satisfying anenergy density of 1/e² of the peak value of the energy density in FIG.36B. The distribution of the energy density of laser light in the casewhere the laser beam is elliptical in shape becomes higher as the centerO of the ellipse is approached. The laser beam shown in FIG. 36A thushas an energy distribution that follows a Gaussian distribution in thecentral axis direction, and a region capable of being judged as having auniform energy density becomes smaller.

The shape of a laser beam in which laser light having the laser beamshape shown in FIG. 36A is synthesized is shown in FIG. 36C. Note thatFIG. 36C shows a case in which laser beams of four laser lights areoverlapped, thus forming one linear shape laser beam, but the number oflaser beams overlapped is not limited to four.

Synthesis is done by making the longitudinal axis of the ellipse foreach laser beam of laser light coincide, and portions of the laser beamsmutually overlapped, thus forming one laser beam 9360. Note that astraight line obtained by connecting the centers O of respectiveellipses is taken as a center axis of the laser beam 9360.

A distribution of the energy density of laser light in the center axis ydirection for the laser beam after synthesis shown in FIG. 36D is shownin FIG. 36D. Note that the laser beam shown in FIG. 36C corresponds to aregion satisfying an energy density of 1/e² of the peak value of theenergy density in FIG. 36B. The energy densities are added in theportions in which the respective laser beams before synthesis areoverlapped. For example, if energy densities E1 and E2 of the overlappedbeams shown in the figures are added, then the calculated value becomenearly equal to the peak value E3 for the beam energy densities, and theenergy densities between the centers O of respective ellipses areleveled.

Note that if E1 and E2 are added, the result ideally becomes equal toE3, but in practice, does not always equal E3. It is possible for adesigner to suitably set a permissible range for the difference betweenthe value of E1 and E2 added, and the value of E3.

If an independent laser beam is used, the energy density follows aGaussian distribution, and therefore it is difficult to irradiate laserlight having a uniform energy density to entire portions in which asemiconductor film is in contact with planarized portions of aninsulating film or portions that become islands. However, by making aplurality of laser lights overlapped such that portions having lowenergy densities mutually complement each other, the uniform energydensity region can be expanded, and the crystallinity of a semiconductorfilm can be increased with good efficiency compared to using anindependent laser light in which a plurality of laser lights are notoverlapped, as can be understood from FIG. 36D.

Note that energy density distributions found by computation in linesegments B-B′ and C-C′ of FIG. 36C are shown in FIGS. 37A and 37B,respectively. Note that, in FIGS. 37A and 37B, regions satisfying anenergy density of 1/e² of the peak value of the laser beams beforesynthesis are taken as a reference. The energy densities in B-B′ andC-C′ when the length of the transverse axis of the laser beams beforesynthesis is 37 μm, the length in the longitudinal direction is 410 μm,and the distance from the center is 192 μm have energy distributions asshown in FIGS. 37A and 37B, respectively. The energy distribution alongB-B′ becomes slightly smaller than that along C-C′, but the two can beseen as being nearly equal. The shape of the synthesized laser beam inthe regions that satisfy an energy density of 1/e² of the peak value ofthe laser beams before synthesis can be expressed as a linear shape.

FIG. 38A is a diagram showing the energy distribution of the synthesizedlaser beam. A region denoted by reference numeral 9361 is a region ofuniform energy density, and a region denoted by reference numeral 9362is a region of low energy density. In FIG. 38, the length of the laserbeam in the central axis direction is taken as W_(TBW), and the lengthin the central axis direction in the region 9361 having uniform energydensity is taken as W_(max). The ratio of the region 9362, which doesnot have uniform energy density and cannot be used in crystallizing thesemiconductor film, to the region 9361, which has uniform energy densityand is capable of being used in crystallization, becomes larger asW_(TBW) becomes large compared to W_(max). Micro-crystals develop andthe crystallinity is poor in the semiconductor film that is irradiatedwith only the non-uniform energy density region 9362. It thereforebecomes necessary to set the layout of the scanning path and the concaveand convex in the insulating film so that regions of the semiconductorfilm that become islands are not overlapped with only the region 9362.This restriction becomes larger if the ratio of the region 9362 to theregion 9361 becomes large. Preventing only the non-uniform energydensity region 9362 from being irradiated to the semiconductor filmformed on depressions or projections of the insulating film by using aslit is therefore effective in making the restriction, which developswhen laying out the scanning path and the concave and convex in theinsulating film, smaller.

It is possible to implement Example 13 in combination with Examples 6 to12.

Example 14

An optical system of a laser irradiation apparatus used in the presentinvention, and the positional relationship between the optical systemand a slit are explained in Example 14.

Laser light having an elliptical shape laser beam follows a Gaussiandistribution for its energy density in a direction that is orthogonal toa scanning direction, and therefore the proportion occupied by regionshaving a low energy density is high compared to laser light having arectangular shape or linear shape laser beam. It is therefore preferablein the present invention that the laser beam of laser light be arectangular shape, or a linear shape, having a relatively uniform energydistribution.

FIG. 39 shows an optical system for a case in which four laser beams aresynthesized to form one laser beam. The optical system shown in FIG. 39has six cylindrical lenses 9417 to 9422. The four laser lights madeincident from the direction of the arrows are made incident to the fourcylindrical lenses 9419 to 9422, respectively. Two laser lights shapedin the cylindrical lenses 9419 and 9421 again have their laser beamsshaped in the cylindrical lens 9417, and are then irradiated to aprocessing piece 9423. On the other hand, two laser lights shaped in thecylindrical lenses 9420 and 9422 again have their laser beams shaped inthe cylindrical lens 9418, and are then irradiated to the processingpiece 9423.

Portions of the respective laser beams of laser light are mutuallyoverlapped, thus forming one synthesized laser beam in the processingpiece 9423.

It is possible for a designer to suitably set the focal length and angleof incidence for each lens, but the focal lengths of the cylindricallenses 9417 and 9418 that are closest to the processing piece 9423 aremade smaller than the focal lengths of the cylindrical lenses 9419 to9422. For example, the focal lengths of the cylindrical lenses 9417 and9418 that are closest to the processing piece 9423 are set to 20 mm, andthe focal lengths of the cylindrical lenses 9419 to 9422 are set to 150mm. Each of the lenses is disposed so that the angle of incidence oflaser light from the cylindrical lenses 9417 and 9418 to the processingpiece 9423 is set to 25° in Example 14, and the angle of incidence oflaser light from the cylindrical lenses 9419 to 9422 to the cylindricallenses 9417 and 9418 is set to 10°. Note that it is preferable that theangle of incidence of the laser light to a substrate be larger than 0°C., preferably from 5 to 30°, in order to exclude returned light andperform uniform irradiation.

An example of synthesis using four laser beams is shown in FIG. 39, andin this case there are four cylindrical lenses corresponding to fourlaser oscillation apparatuses, and two cylindrical lenses correspondingto the four cylindrical lenses. The number of laser beams for synthesisis not limited to this number, and the number of laser beams forsynthesis may be equal to or greater than 2, and equal to or less than8. For cases of synthesis using n laser beams (where n=2, 4, 6, 8),there are n cylindrical lenses corresponding to n laser oscillationapparatus and n/2 cylindrical lenses corresponding to the n cylindricallenses. For cases of synthesis using n laser beams (where n=3, 5, 7),there are n cylindrical lenses corresponding to n laser oscillationapparatuses, and (n+1)/2 cylindrical lenses corresponding to the ncylindrical lenses.

If the locations for placing optical systems, interference, and the likeare considered when five or more laser beams overlapped with each other,it is preferable that a fifth and subsequent laser lights be irradiatedfrom the opposite side of the substrate, and it is also necessary toform a slit on the opposite side of the substrate in this case. Further,it is necessary that the substrate have transparency.

Note that it is preferable that the angle of incidence with respect tothe substrate be maintained to be greater than 0°, and less than 90°, inorder to prevent returned light from returning along the light pathinitially followed.

Further, a surface containing a short edge, or a surface containing along edge, is defined as an incident surface when the surface is avertical flat surface of the irradiation surface and the shape of eachbeam before synthesis is chosen as a rectangular shape. In order toachieve uniform laser light irradiation, it is preferable that an angleof incidence θ of laser light satisfy θ≧arctan (W/2d) when the length ofthe short side or the long side contained in the incident surface is W,and the thickness of the substrate having transparency with respect tothe laser light and disposed in the irradiation surface is d. Thisargument is necessary for each of the laser lights before synthesis.Note that, when the laser light path is not on the incident surface, theangle of incidence of the laser light path projected onto the incidentsurface is set to θ. Light reflected by the front surface of thesubstrate and light reflected from the back surface of the substrate donot interfere provided that the laser light is made incident at theincident angle θ, and uniform laser light irradiation can be performed.The aforementioned argument was considered for a substrate index ofrefraction equal to 1. In practice, substrates often have an index ofrefraction of approximately 1.5, and when taking this value intoaccount, a calculated value that is larger than the angle calculated bythe aforementioned argument is obtained. However, the energy at bothends in the longitudinal direction of the beam spot is attenuated, andtherefore the influence of interference in these portions is little, anda sufficient interference attenuation effect can be obtained by theaforementioned calculated value. The above inequality with respect to θis applied to substrates that do have transparency with respect to thelaser beam.

Note that the optical system of the laser irradiation apparatus used inthe present invention is not limited to the structure shown in Example14.

Further, excimer lasers are typical as gas lasers, and slab lasers aretypical as solid state lasers, which can provide a rectangular shape orlinear shape laser beam without combination of a plurality of laserbeams. These lasers may also be used in the present invention. Further,it is also possible to form a linear shape or rectangular shape laserbeam having uniform energy density by using an optical fiber.

It is possible to implement Example 14 in combination with Examples 6 to13.

Example 15

The relationship between the distance between the centers of each laserbeam, and the energy density when the laser beams are overlapped isexplained in Example 15.

The energy densities in the central axis direction of each laser beamare shown by solid lines, and the energy density of a synthesized laserbeam is shown by a dashed line in FIG. 40. The value of the energydensity in the central axis direction of the laser beams generallyfollow a Gaussian distribution.

The distance between each peak in the beam spots before synthesis istaken as X when the distance in the central axis direction satisfies anenergy density equal to or greater than 1/e² of the peak value. Further,an increase portion in the peak value in the synthesized beam spot withrespect to the average value of the peak value and the valley valueafter synthesis is taken as Y. The relationship between X and Y found bysimulation is shown in FIG. 41. Note that Y is expressed as a percentagein FIG. 41.

The energy difference Y is expressed in FIG. 41 by the approximateequation of Equation 1 below.Y=60−293X+340X ² (where X is the larger of the two solutions)  [Equation1]

In accordance with Equation 1, it is understood that X may be set suchthat X≅0.584 in the case where it is desired to set the energydifference on the order of 5%, for example. Ideally Y=0 becomes true,but the beam spot length becomes shorter, and therefore X may bedetermined in balance with the throughput.

A permissible range for Y is explained next. A distribution of theoutput (W) of a YVO₄ laser with respect to beam width in the centralaxis direction for a case in which the laser beam has an ellipticalshape is shown in FIG. 42. A region shown by inclined lines is a regionhaving an output energy necessary in order to obtain satisfactorycrystallinity, and it is understood that the output energy of thesynthesized laser light may be kept within a range from 3.5 to 6 W.

The energy difference Y at which satisfactory crystallinity can beobtained becomes a maximum when the maximum and minimum values of theoutput energy of the beam spot after synthesis are very close to theoutput energy range necessary to obtain satisfactory crystallinity.Therefore, the energy difference Y becomes ±26.3% in the case of FIG.42, and it is understood that good crystallinity can be obtained byholding the energy difference Y within the aforementioned range.

Note that the range of output energy necessary in order to obtain goodcrystallinity changes by the judgment of what constitutes goodcrystallinity, and further, the output energy distribution changes dueto the laser beam shape, and therefore it is not always necessary thatthe permissible range of the energy difference Y be the above statedvalue. A designer may suitably set the output energy range necessary inorder to obtain good crystallinity, and it is necessary to set thepermissible range of the energy difference Y from the output energydistribution of the laser used.

It is possible to implement Example 15 in combination with Examples 6 to14.

Example 16

The present invention can be applied to various semiconductor devices,and an Example of a display panel manufactured based on Examples 6 to 10is explained using FIGS. 43 and 44.

A pixel portion 9902, gate signal driver circuits 9901 a and 9901 b, adata signal driver circuit 9901 c, an input-output terminal portion9908, and a wiring or wiring group 9904 are prepared on a substrate 9901in FIG. 43. A sealed pattern 9905 may be partially overlapped with thegate signal driver circuits 9901 a and 9901 b, the data signal drivercircuit 9901 c, and the wiring or wiring group 9904 that connects thedriver circuit portion and the input terminals. The surface area of aframe region of the display panel (region in the periphery of the pixelportion) can thus be made smaller. An FPC 9903 is fixed to theinput-output terminal portion 9908.

The present invention can be used in active elements constituting thepixel portion 9902, the gate signal driver circuits 9901 a and 9901 b,and the data signal driver circuit 9901 c.

FIG. 44 is an example showing the structure of one pixel of the pixelportion 9902 shown by FIG. 43. A pixel of a light emitting device, onesemiconductor device of the present invention, is explained in Example16. Note that the term light emitting device is a general term fordisplay panels in which light emitting elements formed on a substrateare enclosed between the substrate and a covering material, and fordisplay modules in which TFTs and the like are mounted to a displaypanel. Note that the light emitting elements have: a layer (lightemitting layer) containing an organic compound in whichelectroluminescence generated by the addition of an electric field canbe obtained; an anode; and a cathode.

Note that the light emitting elements used in Example 16 can have formsin which hole injecting layers, electron injecting layers, holetransporting layers, electron transporting layers, and the like areindependent inorganic compounds, or are formed by materials in which aninorganic compound is mixed into an organic compound. Further, portionsof these layers may also be mutually mixed together.

Reference numeral 9801 denotes a TFT (switching TFT) used as a switchingelement for controlling the input of video signals input to the pixel,and reference numeral 9802 denotes a TFT (driver TFT) for supplyingcurrent to a pixel electrode based on information in the video signals.

The switching TFT 9801 has an active layer 9803 having a plurality ofchannel formation regions with channel widths on the order of 1 to 2 μm,a gate insulating film (not shown), and a gate electrode 9805 that is aportion of a gate line 9804. The switching TFT 9801 controls switchingby selection signals input from the gate signal driver circuits 9901 aand 9901 b to the gate line 9804.

One region, either a source region or a drain region, of the activelayer 9803 of the switching TFT 9801 is connected to a signal line 9806to which the video signals are input by the data signal driver circuit9901 c, and the other region is connected to a wiring 9807 used forconnecting to another element.

Reference numeral 9820 denotes a projection of a base film used duringformation of the active layer 9803.

On the other hand, the driver TFT 9802 has an active layer 9808 having aplurality of channel formation regions with channel widths on the orderof 1 to 2 μm, a gate insulating film (not shown) and a gate electrode9810 that is a portion of a capacitor wiring 9809.

One region, either a source region or a drain region of the active layer9808 of the driver TFT 9802 is connected to an power source line 9811,and the other region is connected to a pixel electrode 9812.

Reference numeral 9821 denotes a projection of the base film used informing the active layer 9808.

Reference numeral 9813 denotes a semiconductor film used for acapacitor, overlapped with the capacitor wiring 9809, sandwiching a gateinsulating film in between. The semiconductor film used for a capacitor9813 is connected to the power source line 9811. A portion in which thesemiconductor film used for a capacitor 9813, the gate insulating film,and the capacitor wiring 9809 are overlapped functions as a capacitorfor storing the gate voltage of the driver TFT 9802. Further, thecapacitor wiring 9809 and the power source line 9811 are overlapped,sandwiching an interlayer insulating film (not shown) in between. It isalso possible for the portion in which the capacitor wiring 9809, theinterlayer insulating film, and the power source line 9811 areoverlapped to function as a capacitor for storing the gate voltage ofthe driver TFT 9802.

Note that the term connection as used in this specification denoteselectrical connections unless otherwise specified.

The directions in which carriers move in the channel formation regionsin the active layer 9803 of the switching TFT 9801 and the active layer9808 of the driver TFT 9802 are each aligned with a laser light scanningdirection as shown by the arrow.

It is preferable that the number of channel formation regions in theactive layer 9808 of the driver TFT 9802 be more than the number ofchannel formation regions of the active layer 9803 of the switching TFT9801. This is because the driver TFT 9802 needs a larger currentcapacity than the switching TFT 19801, and because the ON current can bemade larger as the number of channel formation regions increases.

Note that although the structure of a TFT substrate used in a lightemitting device is explained in Example 16, a liquid crystal displaydevice can also be manufactured by using the manufacturing processes ofExample 16.

It is possible to implement Example 16 by being freely combined withExamples 6 to 10.

Example 17

TFTs in a semiconductor device of the present invention have superiorcrystallinity in their channel formation regions, and therefore circuitsformed by elements that normally use single crystal silicon, for exampleCPUs that use LSIs, all types of storage elements of logic circuits (forexample SRAM), counter circuits, logical divider circuits, and the likecan be formed.

The minimum dimensions for ultra LSI are approaching the sub-micronregion, and partial element three-dimensionalization is necessary inorder to aim for further high integration. The structure of asemiconductor device of the present invention having a stack structureis explained in Example 17.

FIG. 46 shows a cross sectional diagram of a semiconductor device ofExample 17. A first insulating film 9701 is formed on a substrate 9700.A first TFT 9702 is formed on the first insulating film 9701. Note thatthe channel width of a channel formation region of the first TFT 9702 ison the order of 1 to 2 microns.

A first interlayer insulating film 9703 is formed so as to cover thefirst TFT 9702, and a first connection wiring 9705, and a wiring 9704that is electrically connected to the first TFT 9702 are formed on thefirst interlayer insulating film 9703.

A second interlayer insulating film 9706 is then formed so as to coverthe wiring 9704 and the first connection wiring 9705. The secondinterlayer insulating film 9706 is formed by using an inorganicinsulating film, and a film in which a substance that absorbs laserlight irradiated in a later step, for example a colored dye or carbon,is mixed into silicon oxide, silicon oxynitride, or the like is used.

If an upper surface of the second interlayer insulating film 9706 isthen polished by using a chemical mechanical polishing method (CMPmethod), then a second insulating film formed later becomes moreplanarized, and the crystallinity of a semiconductor film formed on thesecond insulating film and crystallized by laser light can be improved.

A second insulating film 9707 is then formed on the second interlayerinsulating film 9706. A second TFT 9708 is then formed on the secondinsulating film 9707. Note that the channel width of a channel formationregion of the second TFT 9709 is on the order of 1 to 2 micron.

A third interlayer insulating film 9709 is formed so as to cover thesecond TFT 9708, and a second connection wiring 9711, and a wiring 9710that is electrically connected to the second TFT 9708 are formed on thethird interlayer insulating film 9709. Note that an embedded wiring(plug) 9712 is formed between the first connection wiring 9705 and thesecond connection wiring 9711 by using a damocene process or the like.

A fourth interlayer insulating film 9713 is then formed so as to coverthe wiring 9710 and the second connection wiring 9711.

Example 17 has a so-called stack structure in which the first TFT 9702and the second TFT 9708 can be overlapped through an interlayerinsulating film. A semiconductor device having a two layer stackstructure is shown in FIG. 46A, but it may also have a stack structureof three or more layers. In this case, an inorganic insulating film,like the second interlayer insulating film 9706, which absorbs laserlight is formed between each layer in order to prevent laser light frombeing irradiated to elements formed in lower layers.

It is possible to have very high integration with a threedimensionalized semiconductor device, and further, wirings forelectrically connecting between each element can be shortened, andtherefore signal delays due to the wiring capacitance can be prevented,and very high speed operation becomes possible.

Note that TFTs which use the present invention can also be used in CAMand RAM coexistent chips as noted in “Fourth New Functional ElementTechnology Symposium Proceedings”, July 1985, p. 205. FIG. 46B is aplanned model for a RAM coexistence chip with content addressable memory(CAM) in which a processor corresponding to memory (RAM) is disposed. Afirst layer is a layer on which a word processor circuit is formed, asecond layer is a layer on which a processor corresponding to RAM of athird layer is formed by using various types of logic circuits, and thethird layer is a layer in which RAM cells are formed. A contentaddressable memory (CAM) is formed by the processor of the second layerand the RAM cells of the third layer. In addition, a fourth layer is aRAM for data (data RAM), and coexists with the content addressablememory formed in the second layer and the third layer.

It is thus possible for the present invention to be applied to varioustypes of three-dimensionalized semiconductor devices.

It is possible to implement Example 17 by being freely combined withExamples 6 to 11.

Example 18

Application to various electronic equipment is possible using asemiconductor device having a TFT manufactured with the presentinvention. Examples of these include portable information terminals(electronic notebooks, mobile computers, mobile phones etc.), videocameras, digital cameras, personal computers, television receivers,mobile telephones, projection type display devices and the like.Specific examples of these electronic equipment are shown in FIGS. 45Ato 45H.

FIG. 45A shows a display device, which is composed of a frame 2001, asupport stand 2002, a display portion 2003, a speaker portion 2004, avideo input terminal 2005, and the like. The display device of thepresent invention is completed using the semiconductor device of thepresent invention in the display portion 2003. The semiconductor deviceis a self-luminous type and thus does not require a backlight.Therefore, a thinner display portion compared to a liquid crystaldisplay can be obtained. Note that the display device includes alldisplay devices for information display such as personal computer, TVbroadcast receiver, and advertisement display.

FIG. 45B shows a digital still camera, which is composed of a main body2101, a display portion 2102, an image receiving portion 2103, operationkeys 2104, an external connection port 2105, a shutter 2106, and thelike. The digital still camera of the present invention is completedusing the semiconductor device of the present invention in the displayportion 2102.

FIG. 45C shows a notebook personal computer, which is composed of a mainbody 2201, a frame 2202, a display device 2203, a keyboard 2204, anexternal connection port 2205, a pointing mouse 2206, and the like. Thenotebook personal computer of the present invention is completed usingthe semiconductor device of the present invention in the display portion2203.

FIG. 45D shows a mobile computer, which is composed of a main body 2301,a display portion 2302, switches 2303, operation keys 2304, an infraredport 2305, and the like. The mobile computer of the present invention iscompleted using the semiconductor device of the present invention in thedisplay portion 2302.

FIG. 45E shows an image reproducing device equipped with a recordingmedium (specifically, DVD playback device), which is composed of a mainbody 2401, a frame 2402, a display device A 2403, a display device B2404, a recording medium (such as a DVD) read-in portion 2405, operationkeys 2406, and a speaker portion 2407, and the like. The display portionA 2403 mainly displays image information, and the display portion B 2404mainly displays character information. Note that household game machinesand the like are included in the category of image reproducing devicesequipped with a recording medium. The image reproducing device of thepresent invention is completed using the semiconductor device of thepresent invention in the display portion A 2403 and the display portionB 2404.

FIG. 45F shows a goggle type display (head mounted display) which iscomposed of a main body 2501, a display portion 2502, and an arm 2503.The goggle type display of the present invention is completed using thesemiconductor device of the present invention in the display portion2502.

FIG. 45G shows a video camera, which is composed of a main body 2601, adisplay portion 2602, a frame 2603, an external connection port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, an audio input portion 2608, operation keys 2609, aneyepiece portion 2610, and the like. The video camera of the presentinvention is completed using the semiconductor device of the presentinvention in the display portion 2602.

FIG. 45H shows a mobile telephone, which is composed of a main body2701, a frame 2702, a display portion 2703, an audio input portion 2704,an audio output portion 2705, operation keys 2706, an externalconnection port 2707, an antenna 2708, and the like. Note that whitecharacters are displayed on a black background in the display portion3703, and thus, the power consumption of the mobile telephone can besuppressed. The mobile telephone of the present invention is completedusing the semiconductor device of the present invention in the displayportion 2703.

As mentioned above, the applicable range of this invention is very wide,and it is possible to apply it to electrical appliances of all fields.It is also possible to implement Example 18 in combination with any ofthe structures shown in Examples 6 to 12.

Example 19

Multi-channel TFTs of semiconductor devices of the present invention cancontrol dispersions in the S value (subthreshold value), mobility,threshold voltage, and the like more than single channel TFTs andmulti-channel TFTs formed by using a semiconductor film which iscrystallized on a flat insulating film.

A frequency distribution of the S value of n-type multi-channel TFTs ofthe present invention is shown in FIG. 47A. The multi-channel TFTs ofthe present invention use semiconductor films that are crystallized byirradiating laser light on an insulating film having depressions andprojections. The width of projections and depressions of the insulatingfilm is 1.25 μm and 1.50 μm, respectively, the TFT channel length is 8μm and the total channel width is 12 μm.

A frequency distribution of the S value of n-type single channel TFTscrystallized on a flat insulating film is shown in FIG. 47B forcomparison. The TFT channel length is 8 μm, and the channel width is 8μm. Further, a frequency distribution of the S value of n-typemulti-channel TFTs crystallized on a flat insulating film is shown inFIG. 47C. The TFT channel length is 8 μm, the total channel width is 12μm, each channel width is 2 μm, and the gap between channels is 2 μm.

The standard deviation σ=15.8 mV/dec in FIG. 47C, and the standarddeviation σ=19.9 mV/dec in FIG. 47C, while the standard deviation σ=8.1mV/dec in FIG. 47A, which is low compared to the other two. It istherefore understood that the n-type multi-channel TFTs of the presentinvention shown in FIG. 47A control dispersions in the S value.

Note that the channel width of the TFTs of FIG. 47B is shorter than thetotal channel width of the TFTs of FIG. 47A. Further, the TFTs of FIG.47C have longer channel widths and longer spacing between channels thanthe TFTs of FIG. 47A. However, even when considering these conditions,the standard deviation of FIG. 47A can be considered to be remarkablysmaller compared to the standard deviations of FIG. 47B and FIG. 47C,and therefore the n-channel TFTs of the present invention can beexpected to have an effect that the S value is controlled.

Next, a frequency distribution of the threshold voltage of n-typemulti-channel TFTs of the present invention is shown in FIG. 48A. Thestructure of the TFTs of FIG. 48A is the same as the case of FIG. 47A.Further, a frequency distribution of the threshold voltage of n-typesingle channel TFTs crystallized on a flat insulating film is shown inFIG. 48B for comparison. The structure of the TFTs of FIG. 48B is thesame as the case of FIG. 48B. Further, a frequency distribution of thethreshold voltage of n-type multi-channel TFTs crystallized on a flatinsulating film is shown in FIG. 48C. The structure of the TFTs of FIG.48C is the same as the case of FIG. 47B.

The standard deviation σ=126 mV in FIG. 48B, and the standard deviationσ=153 mV in FIG. 48C, while the standard deviation σ=80 mV in FIG. 48A,which is low compared to the other two. It is therefore understood thatthe n-type multi-channel TFTs of the present invention shown in FIG. 48Acontrol dispersions in the threshold voltage.

Note that the channel width of the TFTs of FIG. 48B is shorter than thetotal channel width of the TFTs of FIG. 48A. Further, the TFTs of FIG.48C have longer channel widths and longer spacing between channels thanthe TFTs of FIG. 48A. However, even when considering these conditions,the standard deviation of FIG. 48A can be considered to be remarkablysmaller compared to the standard deviations of FIG. 48B and FIG. 48C,and therefore the n-channel TFTs of the present invention can beexpected to have an effect that the threshold voltage is controlled.

Next, a frequency distribution of the mobility of n-type multi-channelTFTs of the present invention is shown in FIG. 49A. The structure of theTFTs of FIG. 49A is the same as the case of FIG. 47A. Further, afrequency distribution of the mobility of n-type single channel TFTscrystallized on a flat insulating film is shown in FIG. 49B forcomparison. The structure of the TFTs of FIG. 49B is the same as thecase of FIG. 47B. Further, a frequency distribution of the mobility ofn-type multi-channel TFTs crystallized on a flat insulating film isshown in FIG. 49C. The structure of the TFTs of FIG. 49C is the same asthe case of FIG. 47C.

The standard deviation σ=7.9% in FIG. 49B, and the standard deviationσ=9.2% in FIG. 49C, while the standard deviation σ=5.2% in FIG. 49A,which is low compared to the other two. It is therefore understood thatthe n-type multi-channel TFTs of the present invention shown in FIG. 49Acontrol dispersions in the mobility. Note that the design values for thechannel width in FIG. 49A are used in calculating the mobility, andtherefore the actual mobility can be considered to be lower on the orderof 20%.

Note that the channel width of the TFTs of FIG. 49B is shorter than thetotal channel width of the TFTs of FIG. 49A. Further, the TFTs of FIG.49C have longer channel widths and longer spacing between channels thanthe TFTs of FIG. 49A. However, even when considering these conditions,the standard deviation of FIG. 49A can be considered to be remarkablysmaller compared to the standard deviations of FIG. 49B and FIG. 49C,and therefore the n-channel TFTs of the present invention can beexpected to have an effect that the mobility is controlled.

Next, a frequency distribution of the threshold voltage of p-typemulti-channel TFTs of the present invention is shown in FIG. 50A. Exceptfor the different polarity, the structure of the TFTs of FIG. 50A is thesame as the case of FIG. 47A. Further, a frequency distribution of thethreshold voltage of p-type single channel TFTs crystallized on a flatinsulating film is shown in FIG. 50B for comparison. Except for thedifferent polarity, the structure of the TFTs of FIG. 50B is the same asthe case of FIG. 47B. Further, a frequency distribution of the thresholdvoltage of p-type multi-channel TFTs crystallized on a flat insulatingfilm is shown in FIG. 50C. Except for the different polarity, thestructure of the TFTs of FIG. 50C is the same as the case of FIG. 47C.

The standard deviation σ=218 mV in FIG. 50B, and the standard deviationσ=144 mV in FIG. 50C, while the standard deviation σ=77 mV in FIG. 50A,which is low compared to the other two. It is therefore understood thatthe p-type multi-channel TFTs of the present invention shown in FIG. 50Acontrol dispersions in the threshold voltage.

Note that the channel width of the TFTs of FIG. 50B is shorter than thetotal channel width of the TFTs of FIG. 50A. Further, the TFTs of FIG.50C have longer channel widths and longer spacing between channels thanthe TFTs of FIG. 50A. However, even when considering these conditions,the standard deviation of FIG. 50A can be considered to be remarkablysmaller compared to the standard deviations of FIG. 50B and FIG. 50C,and therefore the p-channel TFTs of the present invention can beexpected to have an effect that the threshold voltage is controlled.

Next, a frequency distribution of the mobility of p-type multi-channelTFTs of the present invention is shown in FIG. 51A. Except for thedifferent polarity, the structure of the TFTs of FIG. 51A is the same asthe case of FIG. 47A. Further, a frequency distribution of the mobilityof p-type single channel TFTs crystallized on a flat insulating film isshown in FIG. 51B for comparison. Except for the different polarity, thestructure of the TFTs of FIG. 51B is the same as the case of FIG. 47B.Further, a frequency distribution of the mobility of p-typemulti-channel TFTs crystallized on a flat insulating film is shown inFIG. 51C. Except for the different polarity, the structure of the TFTsof FIG. 51C is the same as the case of FIG. 47C.

The standard deviation σ=7.6% in FIG. 51B, and the standard deviationσ=5.9% in FIG. 51C, while the standard deviation σ=4.6% in FIG. 51A,which is low compared to the other two. It is therefore understood thatthe p-type multi-channel TFTs of the present invention shown in FIG. 51Acontrol dispersions in the mobility. Note that the design values for thechannel width in FIG. 49A are used in calculating the mobility, andtherefore the actual mobility can be considered to be lower on the orderof 20%.

Note that the channel width of the TFTs of FIG. 51B is shorter than thetotal channel width of the TFTs of FIG. 51A. Further, the TFTs of FIG.51C have longer channel widths and longer spacing between channels thanthe TFTs of FIG. 51A. However, even when considering these conditions,the standard deviation of FIG. 51A can be considered to be remarkablysmaller compared to the standard deviations of FIG. 51B and FIG. 51C,and therefore the p-channel TFTs of the present invention can beexpected to have an effect that the mobility is controlled.

As shown in FIGS. 47A to 51C, the multi-channel TFTs of the presentinvention have an effect that dispersions in TFT characteristics aresuppressed. Compared to single channel TFTs and multi-channel TFTs inwhich crystallization is performed on a flat insulating film, thecrystal orientation of each channel easily rotates with themulti-channel TFTs of the present invention, and therefore variouscrystal orientations are included. It can therefore be considered thatdispersions in the characteristics caused by crystal orientation areeasily flattened.

Distortions generated along with crystallization can be made toaggregate in regions outside an opening region by melting andcrystallizing a semiconductor film so that it fills the opening regionformed on an insulating surface. That is, a crystalline semiconductorfilm formed so as to fill the opening portion can be freed fromdistortions.

In other words, by forming an opening portion in a base insulating filmand forming a semiconductor so that it fills the opening portion in amelting and crystallization process during crystallization of anon-single crystal semiconductor film by irradiating continuous wavelaser light, distortions, crystal grain boundaries, and crystalsub-boundaries that accompany crystallization can be made to aggregatein regions other than the opening portion. It becomes possible toincrease high speed current driver performance, and it becomes possibleto increase element reliability, by then forming TFTs so that theirchannel formation regions are disposed in the crystalline semiconductorfilm of the opening portion.

With the present invention, the formation of grain boundaries in the TFTchannel formation regions can be prevented by actively usingsemiconductor films located on depressions of a base film as TFT activelayers, and conspicuous drops in TFT mobility, decreases in the ONcurrent, and increases in the OFF current, all due to grain boundaries,can be prevented. Note that a designer can suitably determine just howmuch is to be removed by patterning adjacent to the edges of theprojections and depressions.

Further, regions in which channel formation regions are overlapped withgate electrodes, sandwiching gate insulating films in between, can bewidely taken by mutually separating a plurality of TFT channel formationregions, and therefore the channel width can be made larger. The ONcurrent is secured by making the channel width larger, and heatgenerated by driving the TFT can be dissipated efficiently.

1. A semiconductor device comprising: an insulating film formed over aninsulating surface, the insulating film having an opening portion; and acrystalline semiconductor film formed over the insulating surface andthe insulating film, wherein the crystalline semiconductor filmcomprises a region that fills the opening portion, and wherein thecrystalline semiconductor film comprises a channel formation region inthe filled region.
 2. A semiconductor device comprising: an insulatingfilm formed over an insulating surface, the insulating film comprisingan opening portion; and a crystalline semiconductor film over theinsulating surface and the insulating film, wherein the crystallinesemiconductor film comprises a region that fills the opening portion,wherein the crystalline semiconductor film comprises a channel formationregion in the filled region, wherein the opening portion extends in alongitudinal direction of the channel formation region, and wherein thedepth of the opening portion is equal to or greater than the thicknessof the crystalline semiconductor film.
 3. A semiconductor devicecomprising: an insulating surface having an opening portion having arectangular shape or a stripe shape; a crystalline semiconductor filmformed in the opening portion; a gate insulating film; and a gateelectrode overlapped with the crystalline semiconductor film with thegate insulating film therebetween.
 4. A semiconductor device comprising:an insulating surface having an opening portion; a crystallinesemiconductor film formed in the opening portion; a gate insulatingfilm; and a gate electrode overlapped with the crystalline semiconductorfilm with the gate insulating film therebetween, wherein the crystallinesemiconductor film comprises a channel formation region, and wherein theopening portion extends in a longitudinal direction of the channelformation region.
 5. A semiconductor device comprising: a base film; athin film transistor comprising: an active layer over the base filmcomprising: two impurity regions; and a plurality of channel formationregions between the two impurity regions; a gate electrode overlappedwith the plurality of channel formation regions; and a gate insulatingfilm between the active layer and the gate electrode; wherein the basefilm has a plurality of projections between each of the plurality ofchannel formation regions; and wherein the plurality of channelformation regions are all separated form each other by the plurality ofprojections.
 6. A semiconductor device according the claim 5, wherein,the plurality of channel formation regions have uniform crystallinity.